Compilation of my kernel gives:

cc -O2 -Werror -Wall -Wmissing-prototypes -Wstrict-prototypes -Wno-main -I../../. -I../../../../../../arch -I../../../../../.. -nostdinc -DMAXUSERS=32 -D_KERNEL -Di386 -c /sw/NetBSD/src/sys/compat/common/uipc_syscalls_43.c
/sw/NetBSD/src/sys/compat/common/uipc_syscalls_43.c: In function `compat_43_sys_recvfrom':
/sw/NetBSD/src/sys/compat/common/uipc_syscalls_43.c:185: `MSG_COMPAT' undeclared (first use in this function)
/sw/NetBSD/src/sys/compat/common/uipc_syscalls_43.c:185: (Each undeclared identifier is reported only once
/sw/NetBSD/src/sys/compat/common/uipc_syscalls_43.c:185: for each function it appears in.)
/sw/NetBSD/src/sys/compat/common/uipc_syscalls_43.c: In function `compat_43_sys_recvmsg':
/sw/NetBSD/src/sys/compat/common/uipc_syscalls_43.c:221: `MSG_COMPAT' undeclared (first use in this function)
/sw/NetBSD/src/sys/compat/common/uipc_syscalls_43.c: In function `compat_43_sys_sendmsg':
/sw/NetBSD/src/sys/compat/common/uipc_syscalls_43.c:296: `MSG_COMPAT' undeclared (first use in this function)
*** Error code 1


The only COMPAT_xxx option for this kernel is COMPAT_13. Inspection of sys/socket.h
reveals that MSG_COMPAT is defined if one of several other compat options is defined:

# if defined(COMPAT_43) || defined(COMPAT_SUNOS) || defined(COMPAT_LINUX) || \
     defined(COMPAT_HPUX) || defined(COMPAT_FREEBSD) || defined(COMPAT_ULTRIX)
#  define COMPAT_OLDSOCK
#  define MSG_COMPAT	0x8000
# endif

uipc_syscalls_43.c itself contains no conditionals.

- Erik