Make configure.ac compatible to Sphinx 1.7+
Make configure.ac compatible to Sphinx 1.7+

In 1.7, Sphinx switched from optparse to argparse, changing the output
format of "sphinx-build --version" from "Sphinx (sphinx-build[23]?)
1.x.y" to "sphinx-build[23]? 1.x.y".

Change the regular expression to match both versions.

ganeti/ganeti,mbakke/ganeti,ganeti/ganeti,mbakke/ganeti,mbakke/ganeti,ganeti/ganeti