The qtversions's toolchain now depends on the actual compiler not mkspec

That is we actually parse the mkspec and evaluate QMAKE_CXX (and a few
other variables) to figure out the correct mkspec. This makes using
custom mkspecs easier and is also cleaner. I also changed mkspec() and
mkspecPath() to behave a little diffrent, essentialy mkspec() will
return only the name (the actual dir name) of the mkspec. That is in
general not sufficient for passing on to qmake. mkspecPath() only
returns the correct path to mkspecs/default.
Hopefully I haven't broken WinCE/Maemo/MinGW.