Don't redefine wxNO_EXCEPTIONS in wx/except.h if it's already defined.

This symbol can be defined on compiler command line already, don't redefine it
in this case to avoid a warning.

Closes #12235.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64993 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/except.h

@@ -24,8 +24,10 @@
 // compilation errors in e.g. wx/scopeguard.h with at least g++ 4
 #if !wxUSE_EXCEPTIONS || \
         (defined(__GNUG__) && !defined(__EXCEPTIONS))
+    #ifndef wxNO_EXCEPTIONS
         #define wxNO_EXCEPTIONS
     #endif
+#endif
 
 #ifdef wxNO_EXCEPTIONS
     #define wxTRY