Generalize WX_ATTRIBUTE_PRINTF to WX_ATTRIBUTE_FORMAT

Allow applying gcc "format" attribute to other functions and do apply it
to wxStrftime().

Also suppress -Wformat-nonliteral inside wxStrftime() itself, as it's
now supposed to be checked when calling it.

include/wx/wxcrt.h

@@ -1052,9 +1052,16 @@ inline wchar_t* wxGetenv(const wxScopedWCharBuffer& name) { return wxCRT_GetenvW
 // ----------------------------------------------------------------------------
 
 #ifndef wxNO_IMPLICIT_WXSTRING_ENCODING
+WX_ATTRIBUTE_FORMAT(__strftime__, 3, 4)
 inline size_t wxStrftime(char *s, size_t max,
                          const wxString& format, const struct tm *tm)
-    { return wxCRT_StrftimeA(s, max, format.mb_str(), tm); }
+    {
+        wxGCC_ONLY_WARNING_SUPPRESS(format-nonliteral)
+
+        return wxCRT_StrftimeA(s, max, format.mb_str(), tm);
+
+        wxGCC_ONLY_WARNING_RESTORE(format-nonliteral)
+    }
 #endif // wxNO_IMPLICIT_WXSTRING_ENCODING
 
 inline size_t wxStrftime(wchar_t *s, size_t max,