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.
To keep the distchecks for the documentation, the documentation has to
be built in the check target instead of distcheck-hook.
While at it, rename doc/.build to doc/_build. This naming is the
default with sphinx 0.6.2.
