Bring back the infrastructure around cling-config

[vgvassilev]

In commit 6c9817f we removed several files and folders including a file called _cling_config.py. Later we readded it in 12e48ca, however we still do not generate the cling-config file at the right spot leading to problems when running the test suite (see the test step in the github actions CI).

This happens because it was handled in cling/setup.py which turned into setup.py but still lacks something to produce a valid cling-config.

PS: once we revive this infrastructure we should replace root-config --cppflags with llvm-config --cxxflags.