Rework r349061: Don't apply guessed dependencies if there is a custom target.

This is still targeting bin/sh cyclic dependency issues.  Only apply
guessed dependencies that are explicitly set for an object (which
gnu/lib/cc/cc_tools needs) and if no custom target exists with its
own dependencies.

This was manifesting as a missing yacc.h in usr.bin/mkesdb_static when
built without -j (or -B). No actual yacc.h dependency ordering was
defined but with -j it got lucky and built fine.

Before r349061 the behavior was different for META_MODE but that logic
difference isn't needed.

X-MFC-With:	r349061
Sponsored by:	DellEMC

share/mk/bsd.dep.mk

@@ -264,7 +264,9 @@ _depfile=	${.OBJDIR}/${_dep_obj}
 # - For meta mode we still need to know which file to depend on to avoid
 #   ambiguous suffix transformation rules from .PATH.  Meta mode does not
 #   use .depend files when filemon is in use.
-${__obj}: ${OBJS_DEPEND_GUESS:N*.h}
+.if !target(${__obj})
+${__obj}: ${OBJS_DEPEND_GUESS}
+.endif
 ${__obj}: ${OBJS_DEPEND_GUESS.${__obj}}
 .endif	# !exists(${_depfile}) || defined(_meta_filemon)
 .endfor