pragma warning not working with "external header warning level" option

Since the migration from 2.8.0 to the dev version 2.8.1, #pragma warning(disable) are not working anymore.
I joined a project which reproduce the bug. Just compile in Debug and see that there is a warning:
```UnusedArg.h(5,29): warning C4100: 'iUnusedArg': unreferenced formal parameter```

Note that these VS C++ options are changed to skip warning in external header include with bracket:

When, include a header which disable warning inside a pre compile header and see that the #pragma warning(disable) is not handle by the compiler.

The problem is not present with Qt Addins 2.8.0