Add cross_compile to qconfig.pri for global advertising

Add cross_compile to qconfig.pri for global advertising

.qmake.cache is not necessarily accessible to other modules which depend on
information about whether we are cross compiling or not. We might as well
advertise this fact globally via the CONFIG variable in qconfig.pri.

Change-Id: I6dee3e6604e5ca1c775c5f9f834fe29b4e27adb8
Reviewed-by: Donald Carr <donald.carr@nokia.com>
Reviewed-by: Girish Ramakrishnan <girish.1.ramakrishnan@nokia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Johannes Zellner <johannes.zellner@nokia.com>