Cause an error during 'make install' if trying to compile with CC.

This is limited to src-tree builds, meaning not extended to ports or other
out-of-tree builds.

This will help ensure that read-only OBJDIRS will be respected at install-time
by causing a more consistent failure for those who don't use a read-only
OBJDIR. It also will cause Jenkins to yell. This is a better solution than
trying to see CC=false as has been attempted and discussed before.

Of course this is only relevant for files generated by CC.

Disable this for META_MODE since it will detect the CFLAGS/command
change and force a rebuild.

Sponsored by: EMC / Isilon Storage Division