Add SPHINX_WARNINGS_AS_ERRORS CMake option to allow warnings to not be
treated as errors (which is still the default). This is useful when
working on documentation that has existing errors.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215634 91177308-0d34-0410-b5e6-96231b3b80d8
Make message about building sphinx documentation with CMake more
informative by stating where the output is going.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215635 91177308-0d34-0410-b5e6-96231b3b80d8