Run Sphinx without the -W flag when building documentation

In "make html", don't use the -W flag with Sphinx. This makes it
possible to create the documentation with Sphinx 1.0 without errors,
as the warning about using old-style C markup isn't turned to an
error.

Don't build documentation in "make check". Instead, add a new make
target "check-doc" to build the documentation with the -W flag.