build: don't reject definition of STREQ

* maint.mk (sc_prohibit_strcmp): Tighten rule: exclude
macro definitions like the one in system.h.
* .x-sc_prohibit_strcmp: Don't exempt src/system.h.

.x-sc_prohibit_strcmp

@@ -1,2 +1 @@
-^src/system\.h
 ChangeLog

maint.mk

@@ -132,7 +132,8 @@ sc_prohibit_atoi_atof:
 # Use STREQ rather than comparing strcmp == 0, or != 0.
 sc_prohibit_strcmp:
 	@grep -nE '! *str''cmp *\(|\<str''cmp *\([^)]+\) *=='		\
-	    $$($(VC_LIST_EXCEPT)) &&					\
+	    $$($(VC_LIST_EXCEPT))					\
+	  | grep -vE ':# *define STREQ\(' &&				\
 	  { echo '$(ME): use STREQ in place of the above uses of str''cmp' \
 		1>&2; exit 1; } || :