Mac: Ensure that C++11 is always used when linking against a static Qt build

If Qt was built with C++11, it links to libc++, and we need all projects
that use Qt to link against the same library.

Technically, we could do QMAKE_LFLAGS += -stdlib=libc++, but that hasn't
been tested enough without also enabling C++11, so we keep the
relationship between the two for now.

Change-Id: Ic628bcbade60cc82f93707f372c2119c24d9dc8a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Liang Qi <liang.qi@digia.com>
Fix QDir::mkpath failing for drive paths on Windows

This fixes a regression introduced in 3e2cd8ef6f1ce4f467. We shouldn't try
to call mkdir for just a drive letter.

Task-number: QTBUG-31862

Change-Id: I56035857aec03f9f1cceef62e82f372486b7737b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>