MOBILITY_INCLUDE may be empty
MOBILITY_INCLUDE may be empty

Don't use MOBILITY_INCLUDE if it's empty.
It is empty when building apps against Mobility for Symbian
using the Qt SDK.

qtproject/qt-mobility,tmcguire/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,qtproject/qt-mobility,enthought/qt-mobility,enthought/qt-mobility,kaltsi/qt-mobility,enthought/qt-mobility,enthought/qt-mobility,tmcguire/qt-mobility,tmcguire/qt-mobility,kaltsi/qt-mobility,kaltsi/qt-mobility,kaltsi/qt-mobility,kaltsi/qt-mobility,tmcguire/qt-mobility,enthought/qt-mobility,enthought/qt-mobility,qtproject/qt-mobility,qtproject/qt-mobility,tmcguire/qt-mobility,qtproject/qt-mobility