gstqmlgl: Fix compilation on Windows with qmake

Hello,

this merge request addresses two issues when building the gstqmlgl plugin on Windows with the pre-built development package of gstreamer. Since there's no need to do a meson build for this particular plugin, a build with qmake can be performed instead, but it is currently failing.

1. The plugin cannot not be built with qmake anymore since the new files gstqtoverlay.cc, qtglrenderer.cc were not in the sources list of the project.
2. The plugin cannot resolve dependencies on Windows with qmake, if a ported version if pkg-config is not installed. This is very probable on Windows, since not everybody has msys or cygwin installed. This merge request extracts the required dependencies from the .pc files and configured the flags manually, if no pkg-config is available on Windows.

I've also updated the README, since the plugin can be built on Windows with Qt in some simple steps

Cheers Dmitriy