Add default path to CFLAGS for OpenGL

As of 6cec75a730f95f1f87e7cbdb5eae5ebae06d93e3 we use pkg-config to
pickup OpenGL. Some OpenGL implementations list additional directories
to be used as include directories in CFLAGS. Up until now, if this has
been empty, then the default path has been used. Now that there may be
additional directories, the default include path is not used, which
causes gl.h to be picked up.

Task-number: QTBUG-43377
Change-Id: I86e995cb8d718724a75d9f3f5713e87451fd7129
Reviewed-by: Simo Fält <simo.falt@theqtcompany.com>
remove unsupported winrt mkspecs

These mkspecs are not supported and no longer compile. Related support in
qmake has also been removed.

Change-Id: I7706dcfa5471e55e2ae3d580d65e9371e2c652d5
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
Reviewed-by: Oliver Wolff <oliver.wolff@theqtcompany.com>