Migrate libxml2 to use upstream checkout when possible
CentOS 6 only provides autoconf up to version 2.68.
Building libxml2 from the repository instead of a source tarball requires autoconf v2.69 or higher.
One of the following needs to happen to allow the change:
- https://gitlab.gnome.org/GNOME/libxml2/issues/50 gets resolved
- CentOS 6 goes end-of-life and support stops after November 30, 2020 and is no longer in the build syustem
- EPEL packages autoconf v2.69 in the same way as autoconf v2.68 was packaged