sci-physics/root: fix Cling library load

It was broken for prefix builds (that's how ROOT is installed in
Gentoo. See bug https://sft.its.cern.ch/jira/browse/ROOT-6410.
app-doc/root-docs: properly generate gif images
sci-physics/root: fix ROOT's CLI commands

Arrange changes required for .credits, .demos and .license to work
properly.
More fix for macos. We need to create the link for both linux and macos.

Package-Manager: portage-2.2.10
Merge remote-tracking branch 'github/master'