maint: address sc_prohibit_always-defined_macros.

* cfg.mk: No longer skip it, except where EXIT_SUCCESS is used
	as a witness for stdlib.h.
	Skip this test when appropriate.
	* data/yacc.c: Drop a note about why EXIT_SUCCESS is defined here.

data/yacc.c

@@ -438,6 +438,7 @@ b4_push_if([], [b4_lac_if([], [[
 #    define YYSTACK_ALLOC alloca
 #    if ! defined _ALLOCA_H && ! defined EXIT_SUCCESS && ]b4_c_modern[
 #     include <stdlib.h> /* INFRINGES ON USER NAME SPACE */
+      /* Use EXIT_SUCCESS as a witness for stdlib.h.  */
 #     ifndef EXIT_SUCCESS
 #      define EXIT_SUCCESS 0
 #     endif