Merge branch 'feature/FS1535-cmake-sourcedir-to-builddir-utilities'

This branch adds CMake macros for copying / symlinking files from the source
directory to the build directory and for adding a symlink back to the corresponding
directory in the source tree into every directory in the build tree.

* feature/FS1535-cmake-sourcedir-to-builddir-utilities:
  [CMake] Do copies as a fall-back if symlinks are not available on the platform
  [Cmake] Add macros for copying files from the source tree to the build tree.
  [CMake] add macro dune_symlink_to_source_files
  [CMake] Add macro dune_symlink_to_source_tree