maint.mk: generalize/improve the tight-scope rule

* top/maint.mk: Emit a warning when the test is skipped.
(_gl_TS_dir): Add $(srcdir)/ prefix.
(_gl_TS_function_match): Simplify, rather than trying
to enumerate common types.  Otherwise, it would fail to match an
"extern unsigned char const *" declaration in idutils.
(_gl_TS_extern): Do not endorse use of "XTERN", but do provide
a way to support use of that type of macro.
(_gl_TS_var_match): Simplify regexp.
(_gl_TS_obj_files): New configurable variable.
(_gl_TS_headers): Likewise.