Define variables in qtcreator.pri for the various
install locations, instead of hardcoding the paths
everywhere where they are needed.

Change-Id: Ia34de711a7f2be317272ede023e2a8b9cdcc30fe
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>