CMake: Separate symlinks creation and installation

[ur4t]

When preforming cmake --install $build_dir --prefix $dest_dir without setting CMAKE_INSTALL_PREFIX in advance, symlinks are created directly at default CMAKE_INSTALL_PREFIX locations, which is not the expected behaviour when $dest_dir is not consistent with CMAKE_INSTALL_PREFIX.

This PR fixes the problem by separating symlink creation and installation, which is a common pattern used by xz and zstd.