Suppress strange -Wunsafe-loop-optimizations in wxString code

The error message

wx/string.h:558:47: error: missed loop optimization, the loop counter may overflow
                    [-Werror=unsafe-loop-optimizations]
       for ( Cache::Element *c = cacheBegin; c != cacheEnd; c++ )
                                             ~~^~~~~~~~~~~

doesn't seem to really make much sense, as it shouldn't overflow here.

include/wx/string.h

@@ -554,6 +554,12 @@ private:
       if ( cacheBegin == NULL )
         return NULL;
 #endif
+
+      // gcc 7 warns about not being able to optimize this loop because of
+      // possible loop variable overflow, really not sure what to do about
+      // this, so just disable this warnings for now
+      wxGCC_ONLY_WARNING_SUPPRESS(unsafe-loop-optimizations)
+
       Cache::Element * const cacheEnd = GetCacheEnd();
       for ( Cache::Element *c = cacheBegin; c != cacheEnd; c++ )
       {
@@ -561,6 +567,8 @@ private:
               return c;
       }
 
+      wxGCC_ONLY_WARNING_RESTORE(unsafe-loop-optimizations)
+
       return NULL;
   }