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
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