CMake improvement for consumption of libsc, p4est and Zlib

[scivision]

NOTE: this may be desired as a squash commit due to back-and-forth implementations that aren't useful to preserve in commit history.

This uses FetchContent and CMake cache variables with Git Submodules to propagate variables from child to parent projects reliably and cleanly. It avoids retesting system capabilities when child projects have already done so e.g. SC_HAVE_UNISTD_H

TODO: I need to fix Thunderegg consumption

[scottaiton]

Couple of issues here:

• The CI is failing since it is trying to run p4est tests and failing
• The geoclaw build is failing since we need the "-cpp" gfortran flag to enable preprocessor directives.
• This breaks the ThunderEgg external build, thus the ci for ThunderEgg is failing.