[!694] Optionally make CMake create relative symlinks

Merge branch 'feature/support-relative-symlink-creation' into 'master'

ref:core/dune-common This patch adds a new option DUNE_SYMLINK_RELATIVE_LINKS
that creates relative instead of absolute symlinks.

See merge request [!694]

  [!694]: gitlab.dune-project.org/core/dune-common/merge_requests/694