configure: Make sphinx-build version test work on recent versions

On Fedora: `/usr/libexec/sphinx-build --version` outputs `sphinx-build
1.7.2`.  In bindir we actually have sphinx-build-2 and sphinx-build-3
(python2 and python3 versions), which output `sphinx-build-2 1.7.2` and
`sphinx-build-3 1.7.2` respectively.  Dunno what version others are
using but at least this change should works for most versions I suppose.

configure.ac

@@ -842,7 +842,7 @@ AC_PATH_PROG(SPHINXBUILD,sphinx-build)
 AC_CACHE_CHECK([for version of sphinx-build], fp_cv_sphinx_version,
 changequote(, )dnl
 [if test -n "$SPHINXBUILD"; then
-  fp_cv_sphinx_version=`"$SPHINXBUILD" --version 2>&1 | sed 's/Sphinx\( (sphinx-build)\)\? v\?\([0-9]\.[0-9]\.[0-9]\)/\2/' | head -n1`;
+  fp_cv_sphinx_version=`"$SPHINXBUILD" --version 2>&1 | sed 's/.* v\?\([0-9]\.[0-9]\.[0-9]\)/\1/' | head -n1`;
 fi;
 changequote([, ])dnl
 ])