Bug 1746991 - Avoid warning noise from __STDC_CONSTANT_MACROS being redefined....

Bug 1746991 - Avoid warning noise from __STDC_CONSTANT_MACROS being redefined. r=firefox-build-system-reviewers,andi

Differential Revision: https://phabricator.services.mozilla.com/D134348

mozilla-config.h.in

@@ -28,7 +28,9 @@
  * with the fprintf function family.
  */
 #define __STDC_LIMIT_MACROS
+#if !defined(__STDC_CONSTANT_MACROS)
 #define __STDC_CONSTANT_MACROS
+#endif
 #if !defined(__STDC_FORMAT_MACROS)
 #define __STDC_FORMAT_MACROS
 #endif