fix build for standard DirectX install paths
fix build for standard DirectX install paths

Evaluate the environment variable DXSDK_DIR at qmake time.
This makes sure, the contents get properly quoted, even if the DirectX
SDK is installed into a path that contains spaces.

Change-Id: Id22f56448eef4d392af77e97f46308f1b486c7c7
Reviewed-by: Oswald Buddenhagen <6a8868beda3e139b5ac22b2534a23d3e6ba81819@digia.com>

jfaust/qtmultimedia,jfaust/qtmultimedia,jfaust/qtmultimedia,sergiusens/qtmultimedia,jfaust/qtmultimedia,nezticle/qtmultimedia,nezticle/qtmultimedia,sergiusens/qtmultimedia,sergiusens/qtmultimedia,nezticle/qtmultimedia,mer-qt/qtmultimedia,sergiusens/qtmultimedia,mer-qt/qtmultimedia,webOS-ports/qtmultimedia,jfaust/qtmultimedia,mer-qt/qtmultimedia,webOS-ports/qtmultimedia,nezticle/qtmultimedia,mer-qt/qtmultimedia,webOS-ports/qtmultimedia,webOS-ports/qtmultimedia,sergiusens/qtmultimedia,webOS-ports/qtmultimedia,mer-qt/qtmultimedia,nezticle/qtmultimedia