maint.mk: more tight_scope improvements

maint.mk: more tight_scope improvements

* top/maint.mk: (_gl_TS_var_match): Use $(_gl_TS_extern) here, too.
(_gl_TS_headers): Define only in if-0'd block.
(_gl_TS_dir): Omit the $(srcdir)/ prefix.  Sometimes we need it,
sometimes we must *not* use it.  Adjust uses accordingly.
(sc_tight_scope): Use much simpler grep-based test to determine
whether we skip this rule.