Add cmake directives for finding Qt

cmake has trouble finding Qt4.8 and reports the following error:
-- Could NOT find Qt4 (missing:  QT_MOC_EXECUTABLE QT_RCC_EXECUTABLE QT_UIC_EXECUTABLE QT_INCLUDE_DIR) (found version "4.7.1")
Workaround specified here:
http://www.cmake.org/pipermail/cmake/2013-May/054847.html