Partial revert "cross compile with mingw": unbreak pkg-config

Partial revert "cross compile with mingw": unbreak pkg-config

This partially reverts commit
07a978d3d4b41739eff1061dc45dbfa737bf78b9.

Despite what the commit message said, pkg-config previously worked
and was useful, particularly for static Qt. Qt itself even installs
its own .pc files.

Note: The mkspec win32-g++-cross had a PKG_CONFIG definition to avoid
using the pkg-config installation on the build machine.

Change-Id: Ia4a8d18cd57f74a00bdb9b6a171d20151978a9cc
Reviewed-by: Qt Doc Bot <qt_docbot@qt-project.org>
Reviewed-by: Peter Kümmel <syntheticpp@gmx.net>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>