Read mkspec from used qt version by reading the delault/qmake.conf file on windows

Currently the mkspec on windows is determined by reading the QMAKESPEC inside .qmake.cache file. Some windows installations doesn't contain .qmake.cache file - in this case it could be determined like inside qtcreator:
On windows:
mkspec/default/qmake.conf, the last line contains QMAKESPEC_ORIGINAL which points to the same as QMAKESPEC inside .qmake.cache. Whenever Qt is reconfigured the default mkspec (and its qmake.conf file) is updated.
On linux/mac:
mkspec/default is a symbolic link to the right mkspec. Just read the target of that link.