PROGS+META_MODE: Avoid rebuilding common sources when recursing.

This could be seen in lib/libkvm/tests where kvm_test_common.o was
a common dependency, but one of the recursed progs had a special
CFLAGS+= -I that changed the build command. This would cause
all recursed builds to rebuild while fighting over the meta file
and object file.

Reported by: Mark Millard
MFC after: 1 week
Sponsored by: Dell EMC Isilon