Fix for GCC compiler warning

modules/juce_box2d/juce_box2d.h

@@ -57,8 +57,10 @@
 #ifdef __GNUC__
  #pragma GCC diagnostic push
  #pragma GCC diagnostic ignored "-Wconversion"
- #if __has_warning("-Wzero-as-null-pointer-constant")
-  #pragma clang diagnostic ignored "-Wzero-as-null-pointer-constant"
+ #if defined (__clang__)
+  #if __has_warning("-Wzero-as-null-pointer-constant")
+   #pragma clang diagnostic ignored "-Wzero-as-null-pointer-constant"
+  #endif
  #endif
 #endif