CMake: Use path manipulation features from qtbase.

The qtbase commit 4ce32f66 (Use new path manipulation functions
from qmake., 2013-01-18) used new qmake functions for path manipulation,
and commits 7ac58d1f (Make cmake packages installed to /usr
non-relocatable., 2013-02-11) and 5aa34962 (Handle usr-move without
forcing absolute paths, 2013-04-17) added handling of symlinks which
would otherwise break relative path calculation.

Port those patches to the qttools cmake files.

Task-number: QTBUG-32570
Change-Id: I7d22609c3a03be4b9197dd030d7e9bd6c2e04731
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>