[PATCH] Allow CMake to build Sphinx documentation

[Dan Liew]

> Thanks! I committed these in r206655, r206660, and r206662.

Thanks.

> I tested the LLVM and clang docs and fixed some issues, but I'm not set up
> to build lld on Linux at the moment.

I tested building lld inside the LLVM source tree and it seemed to work fine.

> This didn't work for me on Windows because sphinx-build isn't an exe.  The
> right fix is probably to prefix the command with ${PYTHON_EXECUTABLE}, but
> that can be separate.

That sounds good to me. I don't have a windows machine setup to build
LLVM so please fix this if you the time.

> I also made the output quiet (-q) and made warnings into errors (-W), since
> this is really what we want in a build.

This seems sensible, especially for buildbots where the progress
information would just clutter up the logs. Although sometimes I quite
like having the non quiet output. We could perhaps have a CMake cache
variable SPHINX_FLAGS that by default contains "-q" which the user can
change if necessary.

-- 
Dan Liew
PhD Student - Imperial College London