TS-2377: install man pages as part of the build

Add options to conf.py to tell us the man pages for each section,
because AFAICT there's no portable way to set automake variables
from shell output commands. We also teach conf.py to test for a
current sphinx version at configure time.

Wire up sphinx to the man page install and test for the sphinx
version pretty aggressively at configure time. Distro packaging
seems prety variable, so I think it's best to be conservative.