CMake should support static builds

[guitargeek]

Moved from JIRA ROOT 6446 because I think public access to the issue can be beneficial in discussing how to address it

configure / make supports make static (fixed for ROOT 6 now, too), which builds libRoot.a and a (mostly) statically linked bin/roota. We need to support that with CMake, too.
Once done, a build in the CI stressing the feature should be added (from https://its.cern.ch/jira/browse/ROOT-6690)

See also these forum posts:

• https://root-forum.cern.ch/t/static-compiling-of-root-v6-xx
• https://root-forum.cern.ch/t/static-builds-of-root-in-reference-to-jira-ticket-root-6446

[dpiparo]

What is the use case of a static ROOT build? Why would it be an advantage?

[guitargeek]

From the forum posts linked above, we can see that there is a clear demand for linking the ROOT libraries statically into other applications, such that the application can be shipped as one binary without shipping ROOT.

IMO that's quite a valid use case, although I admit it has not been discussed again in the last 7 years. Maybe @pcanal knows more about other requests in this direction, since he was recently commenting about it #19769 (comment)?