Suppress -Wsuggest-override warnings in user code for gcc too

This was already done for clang -Winconsistent-missing-override, but
gcc has a similar warning since 5.1 and, moreover, latest versions of
clang support this gcc warning as well, so add a special macro which
handles both compilers and use it in all wx macros defining virtual
functions instead of just disabling one of the clang warnings.

Closes https://github.com/wxWidgets/wxWidgets/pull/2000

include/wx/richtext/richtextuicustomization.h

@@ -103,13 +103,13 @@ protected:
 /// of the formatting dialog.
 
 #define DECLARE_HELP_PROVISION() \
-    wxCLANG_WARNING_SUPPRESS(inconsistent-missing-override) \
+    wxWARNING_SUPPRESS_MISSING_OVERRIDE() \
     virtual long GetHelpId() const { return sm_helpInfo.GetHelpId(); } \
     virtual void SetHelpId(long id) { sm_helpInfo.SetHelpId(id); } \
     virtual wxRichTextUICustomization* GetUICustomization() const { return sm_helpInfo.GetUICustomization(); } \
     virtual void SetUICustomization(wxRichTextUICustomization* customization) { sm_helpInfo.SetUICustomization(customization); } \
     virtual bool ShowHelp(wxWindow* win) { return sm_helpInfo.ShowHelp(win); } \
-    wxCLANG_WARNING_RESTORE(inconsistent-missing-override) \
+    wxWARNING_RESTORE_MISSING_OVERRIDE() \
 public: \
     static wxRichTextHelpInfo& GetHelpInfo() { return sm_helpInfo; }\
 protected: \