Cf !84. We don't want to just replace the old path without versionned
python, because I assume it has been set for a reason. So there may be
machines with a versionned python path, others without. Let's test both
on Windows.
In particular, when header check fails, Python option is disabled.
Also let's use pkg-config to search for the Python include directory.
The reason is that include/python{version} is not always
true. On my system in particular, python3.7 was in `include/python3.7m`.
