qt5: revert two commits by Ossi (e5024c and 4a35f2)

.. that causes qt_tool_*.pri to have Windows dir seps
e.g. qt_tool_moc.pri:

QT_TOOL.moc.binary = C:\repo\mingw-w64-qt5\...

.. which causes a failure when these are used by make as
happens with qwebkit when it calls perl with
--preprocessor '$QT_TOOL_MOC -E'

Probably two variants are needed, e.g. '_SYS' and '' where '_SYS'
means called directly by the system and '' means called by make.

I need to discuss these commits with Ossi as I think the first one
is completely wrong (system on Windows is not a shell at all ever,
it's simply CreateProcess ..). The second one, I don't think is much
better either.

Note, the patches 0028 and 0029 aren't literal reversions as
there's some other changes in-between that caused me to hand
edit these patches so the apply.