Merge branch 'marc/298-tests-fail-when-symlink-support-is-disabled' into 'master'

Resolve "Tests fail when symlink support is disabled"

This MR does several things:
1. `SUPPORT_SYMLINKS` is now disabled by default. It didn't do much in the first place and only affects incomplete code. The corresponding README entry has been removed. The only thing it does support is accessing GekkoFS from a foreign namespace via a symbolic link. However, only `stat` seems to be working.
2. `GKFS_FOLLOW_EXTERNAL_SYMLINKS` was also disabled by default in this [MR](https://storage.bsc.es/gitlab/hpc/gekkofs/-/merge_requests/183). This caused the `test_symlink` to fail as it tested external symlinks into GekkoFS, not actual symlinks within GekkoFS. This can only be done via `lstat()` for each component of the path which is a performance risk under certain circumstances. Overall, this is the relevant CMake variable for the test.
3. Unify code formatting for CMake files.

Closes #298

Closes #298

See merge request !198