Download ginac/cln archive instead of using git repo

[volkm]

The git repositories for Ginac and CLN cannot be cloned since 2 weeks and this is the second time there is an issue with the repo. This PR fixes the issue by using the direct download instead.
Disadvantages are:

• only the latest version is supported for download which means a timely update is required for a new version, and
• the zipped archive of CLN has no CMake support

[sjunges]

Thanks for taking care of this. Maybe add a comment why the git repos have been commented out?

LGTM.

[volkm]

Thanks for the quick feedback and the suggestion for better documentation.