allow qbs to be built without rpath

Some Linux distros discourage the use of rpath.
Specify CONFIG+=disable_rpath to turn off rpath usage on Linux and
OS X.

Task-number: QBS-309

Change-Id: I3658470aef525f5406d728c48ab2e6f86a3c62cd
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>