maint.mk: avoid using a temporary file in the always-defined-macros check

maint.mk: avoid using a temporary file in the always-defined-macros check

* top/maint.mk (.re-defmac): Remove rule.
(gl_trap_): Remove definition.
(sc_prohibit_always-defined_macros): Rewrite not to create and
depend on a temporary file.  Instead, depend on GNU grep's ability
to read a list of regular expressions from stdin when given "-f -".