Be more tolerant of share/mk and kern.mk mismatch

When building out-of-tree modules, it appears that the system share/mk
is used, but sys/conf/kern.mk is used. That results in MK_INIT_ALL_ZERO
being undefined. In the interest of maximum compatability, check
that MK_INIT_ALL_* and COMPILER_FEATURES are defined before comparing
their values.

Reported by: mmacy
Sponsored by: DARPA