boost/python included despite BUILD_PYTHON_MODULE=OFF

[G.A. vd. Hoorn]

As per subject.

All of the Boost Python related bits of CMakeLists.txt are skipped when BUILD_PYTHON_MODULE is set to OFF, but offset_sorter.hpp seems to unconditionally include boost/python.hpp (see here).

Later on, there is another issue with boost::python::list offset_list_py().

Making the #include and function definition conditional on BUILD_PYTHON_LIB seems to fix this, but I'm unsure as to the consequences of just disabling things.

[G.A. vd. Hoorn]

island_sorter.hpp also unconditionally includes boost/python.hpp.