CMake: Use canonical source directory when talking to cmake server-mode

Use the canonical form of the source directory when talking to cmake.

Cmake will save the canonical source directory in its settings and will
fail if that is not passed in again.

Change-Id: I1ea578053c8d5136d09b3f503443bd0188ebca4f
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>