CMake option to use a system installed sc

[sandro-elsweijer]

Add CMake option to use a system installed SC

Proposed changes:
For our new t8code CMake CI we need the option to build p4est with a system installed SC version.
This option already exists in p4ests autotools build system but does not exist in the CMake build system.
This PR adds this option. The standard behavior of p4ests CMake build system is not altered since the standard policy is to use the SC which ships with p4est.

[sandro-elsweijer]

Whoopsie, I did not see the other compile option and merged them now.

[cburstedde]

Thanks so much! We're in the middle of merging a few CMake updates in libsc and will then align p4est to work the same way as much as possible. Let's keep an eye on these merges to make sure your code works cleanly with those.

[scivision]

This PR should be rebased on develop p4est branch to make the CI pass

[cburstedde]

Thanks again! I'd be hoping that this runs fine on top of the latest develop branch.

[sandro-elsweijer]

@cburstedde Done, at least locally all tests pass with the libsc shipped by p4est and also with a locally installed libsc (:

[cburstedde]

Thanks, and welcome to the list of developers! :)

As a follow-up, please consider #312 and let us know if that affects this update.

[cburstedde]

Ah, btw., if (not necessarily for this one, but in the future) you might add a line to doc/release_notes.txt.