cmake: relocatable package for recent CMake versions

[nim65s]

Hi,

This enable some recent CMake features when they are available:

1. stop linking to libpython.so as this is not necessary. And actually, it's not even a problem if this lib is not available on the build or run system. Plus it get stripped anyway when --as-needed is activated.
2. provide only references to cmake targets instead of hardcoded absolute path in exported modules.

Enabling 2. allows to move around the installed project and / or its dependencies without issues, which is a required thing when we build / install on a dynamic location to ensure a proper isolation.

This PR shouldn't change anything for older systems.

[nim65s]

Ok, I'll move that into the JRL cmakemodule

[jcarpent]

Ok, I'll move that into the JRL cmakemodule

Perfect. It will simplify future adjustments.

[nim65s]

CI looks OK, except for the job requiring jrl-umi3218/jrl-cmakemodules#518 to be merged first