Disable gcc warnings by adding pragmas and re-enable headersclean.

- IAxServerBase inherits IUnknown which does not have a virtual destructor.
- QAxFactory::metaObject(const QString &) hides QObject::metaObject()
  (false positive).

Task-number: QTBUG-45666
Change-Id: I8d1e69903ba6edde84ae9d4b64ee2e6088efad8b
Reviewed-by: Andy Shaw <andy.shaw@digia.com>