I recalled Marshall had already made this change. The change is committed in e3f89a989a23b9bfcb9f9d01172cebb63db627e1.
libc++ Documentation ==================== The libc++ documentation is written using the Sphinx documentation generator. It is currently tested with Sphinx 1.1.3. To build the documents into html configure libc++ with the following cmake options: * -DLLVM_ENABLE_SPHINX=ON * -DLIBCXX_INCLUDE_DOCS=ON After configuring libc++ with these options the make rule `docs-libcxx-html` should be available.