Skip reading depend files with -V unless looking up a depend variable.

This speeds up some simple -V lookups significantly.

Reported by:	bde
MFC after:	2 weeks
Sponsored by:	Dell EMC

sys/conf/kern.post.mk

@@ -201,6 +201,7 @@ _meta_filemon=	1
 # since it will track dependencies itself.  OBJS_DEPEND_GUESS is still used
 # for _meta_filemon but not for _SKIP_DEPEND.
 .if !defined(NO_SKIP_DEPEND) && (make(*obj) || \
+    (!empty(.MAKEFLAGS:M-V) && empty(.MAKEFLAGS:M*DEP*)) || \
     ${.TARGETS:M*clean*} == ${.TARGETS} || \
     ${.TARGETS:M*install*} == ${.TARGETS})
 _SKIP_DEPEND=	1