MKCROSSGDB=yes broken in new gdb?

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

$ time nice -n +10 env -i PATH=/bin:/usr/bin ./build.sh -O ../obj.arm64 -T ../obj.arm64/tooldir -U -u -m evbarm64 -j4 -N1 -V MKCROSSGDB=yes -V MKDEBUG=yes -V USE_PIGZGZIP=yes release
[...]
In file included from /home/riastradh/netbsd/current/src/tools/gdb/../../external/gpl3/gdb/dist/gdb/gdbtypes.h:58:0,
                 from /home/riastradh/netbsd/current/src/tools/gdb/../../external/gpl3/gdb/dist/gdb/symtab.h:28,
                 from /home/riastradh/netbsd/current/src/tools/gdb/../../external/gpl3/gdb/dist/gdb/d-lang.c:21:
/home/riastradh/netbsd/current/src/tools/gdb/../../external/gpl3/gdb/dist/gdb/gmp-utils.h:29:10: fatal error: gmp.h: No such file or directory
 #include <gmp.h>
          ^~~~~~~
compilation terminated.
nbgmake[1]: *** [d-lang.o] Error 1