bsd.prog.mk: Skip warnings if compiler.mk was not included

We emit a warning if src.conf options WITH_RETPOLINE or INIT_ALL are
in use, and the compiler or linker does not support the feature.

For targets like cleandir we skip including bsd.compiler.mk, which
leaves COMPILER_FEATURES unset and spurious warnings were emitted in
early buildworld stages, if these options are in use.

Commit 0837b2ebd570 ("pkg: Die if make -V BRANCH fails") added -W to
usr.sbin/pkg's make flags, causing it to exit on warnings. As a result
buildworld failed when these src.conf options were in use.

Clean this up by omitting the warning if the magic
_NO_INCLUDE_COMPILERMK flag is set, which indicates that
COMPILER_FEATURES is not set.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53501

(cherry picked from commit e4f2a350955406ffd58dd3f0bb7610df0f36d647)