Fix warning about undefined __cplusplus in wx/defs.h

Check that __cplusplus is defined before testing its value to avoid warnings
when compiling C code, e.g. regex library.

Notice that in master a check for __cplusplus covering a bigger part of this
file was added to solve the same problem, but it doesn't seem like a good idea
to stop defining other symbols which had been previously defined in 3.0, so
keep this check as narrow as possible here.

include/wx/defs.h

@@ -277,12 +277,13 @@ typedef short int WXTYPE;
 #endif /*  HAVE_EXPLICIT/!HAVE_EXPLICIT */
 
 /* wxFALLTHROUGH is used to notate explicit fallthroughs in switch statements */
-
+#ifdef __cplusplus
 #if __cplusplus >= 201103L && defined(__has_warning)
     #if WX_HAS_CLANG_FEATURE(cxx_attributes)
         #define wxFALLTHROUGH [[clang::fallthrough]]
     #endif
 #endif
+#endif /* __cplusplus */
 
 #ifndef wxFALLTHROUGH
     #define wxFALLTHROUGH ((void)0)