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 0837b2ebd5 ("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

share/mk/bsd.prog.mk

@@ -49,9 +49,11 @@ CXXFLAGS+= -mretpoline
 LDFLAGS+= -Wl,-zretpolineplt
 .endif
 .else
+.if !defined(_NO_INCLUDE_COMPILERMK)
 .warning Retpoline requested but not supported by compiler or linker
 .endif
 .endif
+.endif
 # LLD sensibly defaults to -znoexecstack, so do the same for BFD
 LDFLAGS.bfd+= -Wl,-znoexecstack
 .if ${MK_BRANCH_PROTECTION} != "no"
@@ -71,9 +73,11 @@ CFLAGS+= -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clan
 CXXFLAGS+= -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
 .endif
 .else
+.if !defined(_NO_INCLUDE_COMPILERMK)
 .warning INIT_ALL (${OPT_INIT_ALL}) requested but not supported by compiler
 .endif
 .endif
+.endif
 
 # Zero used registers on return (mitigate some ROP)
 .if ${MK_ZEROREGS} != "no"