examples_data_path should not need SIRF_install_path

[KrisThielemans]

ideally we don't need any env variables at all. This would be solvable by letting CMake create a file with a function for the location install_path or similar (i.e. just return CMAKE_INSTALL_PREFIX), see e.g. https://github.com/SyneRBI/SIRF/blob/master/cmake/version.h.in