maint: avoid doubled name in syntax check failures

maint: avoid doubled name in syntax check failures

Based on the similar gnulib commit 96ad9077.  The use of
$(_sc_search_regexp) already injects $(ME) into any output
messages, so a failure of these rules would look like this,
pre-patch:

maint.mk: maint.mk: use virStrToLong_*, not strtol variants

* cfg.mk (sc_prohibit_strncmp, sc_prohibit_strtol)
(sc_libvirt_unmarked_diagnostics): Drop redundant $(ME).