Rather than looking for pygobject-3.0 with pkg-config, run an actual
test in python, loading the 'gi' module.
I also added a version parameter making it possible to even check for a
required version of PyGObject, though it is currently unused (as I have
no idea if we have a minimum requirement for our plug-ins to work well).
This is an improvement of commit 8e938e0960 as I realized that we don't
need any development files, therefore I don't think that searching with
pkg-config was the right idea in our case.
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`.
Jehan