MSYS: Force relpath and outpath into same pathing namespace.

MSYS bash (and other msys-compiled executables) perform path
transformation much like Cygwin. This causes syncqt path
substitution to not work correctly, so use 'pwd -W' to get
the Windows version (though with forward slashes) of both
relpath and outpath.

Change-Id: I808e3ef9206ed5f5bd8b6879d12afe664e589e0c
Reviewed-by: Alvaro Burnett <alvaroburnett@gmail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>