Include python to have python-helpers for python_install_on_site.

as eigenpy doesn't need the interpreter, rely on it is not enough

Otherwise we can't include its fwd.hh

On 16.04, fix:
…/dynamic-graph-python-4.0.0/include/dynamic-graph/python/module.hh:47:26: error: 'exposeEntity' function uses 'auto' type specifier without trailing return type
inline auto exposeEntity() {
^

Remove sphinx and subdirectory doc from CMakeLists.txt

* [CMake] Add Cmake option to deactivate suffix of so files.


Co-authored-by: Joseph Mirabel <josephmirabel@gmail.com>

This should be temporary, until either:
- we find a proper solution in the submodule
- we are sure that no project use the .pc anymore