It should now be easier to read, and more featureful. Among other
tweaks, we now print a summary of dependencies and build options, and
the documentation is generated with more sensible breadcrumbs and
builds properly with Sphinx 1.3.

REVIEW: 126075

This is deliberately modelled very closely on CMake's documentation
system.  It's a hefty patch, because it involved changing all the
documentation to be in reStructuredText format.  I also cleaned up the
copyright/license statements at the same time.

Note that the find modules contain the full license, due to the fact
that ecm_use_find_module() copies them out of the ECM distribution.