Subprojects can use meson.add_dist_script() if meson.version() >= 0.58.0

* meson.build:
* skeletonmm/doc/reference/meson.build:
* skeletonmm/meson.build:
* skeletonmm/skeleton/skeletonmm/meson.build:
Call add_dist_script() in a subproject, if meson.version() >= 0.58.0.
* util/build_scripts/dist-build-scripts.py:
* util/build_scripts/dist-changelog.py:
* util/build_scripts/doc-reference.py:
* util/build_scripts/generate-binding.py:
* util/meson_aux/extra-dist-cmd.py:
Use MESON_PROJECT_DIST_ROOT if it exists, else MESON_DIST_ROOT.
It exists if meson.version() >= 0.58.0.