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.