Update to 1.21.

The project is now hosted on GitLab, which is better than Google Drive from
a porting perspective. We can now drop a few workarounds and patches that
have been upstreamed.

Unfortunately this release is being fetched directly from GitLab and thus we
need to run gtkdocize and autoreconf ourselves as there is no actual release
tarball.

Submitted by: Tobias Berner <tcberner@gmail.com>