meson_vapi_link.py doesn't construct vapidir correctly if DESTDIR is set
I think because os.path.join() discards all previous path segments when an absolute path appears, and sys.argv[1] is datadir as an absolute path, DESTDIR is always being ignored.
e.g. see https://travis-ci.org/jon-turney/meson-corpus-test/jobs/586725200#L549
Note the error "FileNotFoundError: [Errno 2] No such file or directory: '/usr/local/share/vala/vapi'", where it's trying to make the links in a (non-existent) directory due to ignoring DESTDIR.