I attempted to build a kernel without COMPAT_43 and was repaid with
this during the make. Simply putting back the COMPAT_43 fixed
it. Something tells me this shouldn't be the case. Should I send-pr this?

making sure the compat library is up to date...
cc -O2 -Werror -I../../. -I../../../../../../arch -I../../../../../.. -DI486_CPU -DI586_CPU -DMACHINE_NONCONTIG -DTIMEZONE=0 -DDST=0 -DSWAPPAGER -DVNODEPAGER -DDEVPAGER -DDIAGNOSTIC -DKTRACE -DCOMPAT_10 -DFFS -DMSDOSFS -DFDESC -DFIFO -DKERNFS -DNULLFS -DPROCFS -DINET -DGENERIC -D_KERNEL -Di386 -c ../../../../../../compat/common/tty_43.c
../../../../../../compat/common/tty_43.c: In function `ttcompat':
../../../../../../compat/common/tty_43.c:96: `TIOCGETP' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:96: (Each undeclared identifier is reported only once
../../../../../../compat/common/tty_43.c:96: for each function it appears in.)
../../../../../../compat/common/tty_43.c:102: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:104: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:104: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:107: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:109: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:110: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:111: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:115: `TIOCSETP' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:116: `TIOCSETN' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:122: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:126: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:130: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:131: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:132: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:138: `TIOCGETC' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:142: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:143: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:144: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:145: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:146: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:147: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:150: `TIOCSETC' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:154: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:155: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:156: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:157: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:158: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:159: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:160: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:164: `TIOCSLTC' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:168: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:169: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:170: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:171: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:172: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:173: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:176: `TIOCGLTC' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:180: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:181: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:182: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:183: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:184: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:185: dereferencing pointer to incomplete type
../../../../../../compat/common/tty_43.c:188: `TIOCLBIS' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:189: `TIOCLBIC' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:190: `TIOCLSET' undeclared (first use this function)
cc1: warnings being treated as errors
../../../../../../compat/common/tty_43.c:206: warning: unreachable code at beginning of switch statement
../../../../../../compat/common/tty_43.c:210: `TIOCLGET' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:216: `OTIOCGETD' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:220: `OTIOCSETD' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:228: `OTIOCCONS' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:232: `TIOCHPCL' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:236: `TIOCGSID' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:246: warning: unreachable code at beginning of switch statement
../../../../../../compat/common/tty_43.c: In function `ttcompatgetflags':
../../../../../../compat/common/tty_43.c:263: `TANDEM' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:265: `CRMOD' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:269: `ODDP' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:271: `EVENP' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:273: `ANYP' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:280: `CBREAK' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:282: `RAW' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:286: `LITOUT' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:286: `PASS8' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:297: `NOHANG' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:299: `XTABS' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:301: `CRTERA' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:301: `CRTBS' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:303: `CRTKIL' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:305: `PRTERA' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:307: `CTLECH' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:309: `DECCTQ' undeclared (first use this function)
../../../../../../compat/common/tty_43.c: In function `ttcompatsetflags':
../../../../../../compat/common/tty_43.c:326: `TANDEM' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:334: `CRMOD' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:341: `XTABS' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:347: `RAW' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:354: `CBREAK' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:358: `ANYP' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:366: `EVENP' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:371: `ODDP' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:379: `LITOUT' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:379: `PASS8' undeclared (first use this function)
../../../../../../compat/common/tty_43.c: In function `ttcompatsetlflags':
../../../../../../compat/common/tty_43.c:414: `PRTERA' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:418: `CRTERA' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:427: `NOHANG' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:431: `CRTKIL' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:435: `CTLECH' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:439: `DECCTQ' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:446: `RAW' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:446: `LITOUT' undeclared (first use this function)
../../../../../../compat/common/tty_43.c:446: `PASS8' undeclared (first use this function)
*** Error code 1

Stop.
*** Error code 1

Stop.