Add a guard for broken SUBDIR.${MK_FOO} use

Check for the variable SUBDIR. and error as it usually means someone
forgot to include src.opts.mk.

This guard from CheriBSD found the bugs in r367655 and r367728.

Reviewed by: bdrewery, arichardson
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D27211