Björn Esser authored Jul 29, 2017 and Brad King committed Aug 02, 2017

Since commit v3.8.0-rc2~28^2~2 (Utilities/Sphinx: Port cmake extension
to Sphinx 1.4, 2017-02-09) we use the `sphinx.version_info` tuple.
However, it was added in Sphinx v1.2 so the check breaks compatibility
with older versions.  Revise our check to assume Sphinx pre-1.2 if the
version tuple does not exist.