Fix linking of qgis against homebrew python.

By applying the patch from this pull request https://github.com/qgis/Quantum-GIS/pull/597

At the moment QGIS prints several warnings on startup due to linking problems with homebrew python. This happens because ```libqgispython.dylib``` is linked against the system python library (check otool -L on the file). This happens because the qgis cmake scripts checks for the existence of the file PYTHON_LIBRARY instead of the existence of the file the variable of the same name points to. This is fixed in the pull request against qgis used as a patch here. 

In addition the variable -DPYTHON_INCLUDE_PATH is added because QGIS uses this one and not 
-DPYTHON_INCLUDE_DIR. According to the documentation this is to surport old versions of CMAKE and may change in the future thus -DPYTHON_INCLUDE_DIR remains for now.