Support more sphinx-build versions in configure script

`sphinx-build` on CentOS
(`python-sphinx10-doc-1.0.8-1.el6.centos.noarch`) responds with this on
`stderr`:
```
$ sphinx-1.0-build --version
Sphinx v1.0.8
Usage: /usr/bin/sphinx-1.0-build [options] sourcedir outdir
[filenames...]
Options: -b <builder> -- builder to use; default is html
         -a        -- write all files; default is to only write new and
changed files
...
Modi:
* without -a and without filenames, write new and changed files.
* with -a, write all files.
* with filenames, write these.
```

This should make it work for the old and the new format.

Test Plan:
```
$ echo "Sphinx (sphinx-build) 1.0.1" | sed 's/Sphinx\(
(sphinx-build)\)\? v\?\([0-9]\.[0-9]\.[0-9]\)/\2/'
1.0.1
$ echo "Sphinx v1.0.1" | sed 's/Sphinx\( (sphinx-build)\)\?
v\?\([0-9]\.[0-9]\.[0-9]\)/\2/'
1.0.1
```

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie, erikd

Differential Revision: https://phabricator.haskell.org/D1361

1 files changed, 1 insertions, 1 deletions

diff --git a/configure.ac b/configure.ac
index 925846a3a9..dabcbcd78a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -775,7 +775,7 @@ AC_PATH_PROG(SPHINXBUILD,sphinx-build)
 AC_CACHE_CHECK([for version of sphinx-build], sphinx_version,
 changequote(, )dnl
 [if test -n "$SPHINXBUILD"; then
-  sphinx_version=`"$SPHINXBUILD" --version | sed 's/Sphinx (sphinx-build) \([0-9]\.[0-9]\.[0-9]\)/\1/'`;
+  sphinx_version=`"$SPHINXBUILD" --version 2>&1 | sed 's/Sphinx\( (sphinx-build)\)\? v\?\([0-9]\.[0-9]\.[0-9]\)/\2/' | head -n1`;
 fi;
 changequote([, ])dnl
 ])