mkqt5bld: Add support for parallel builds with mingw

Allow mingw32-make to use -jN when building on Windows.

Change-Id: Ib499c995a99ab5faa5a895368159dcf448925bec
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>