cmake: Add ordering dep between HTML Sphinx docs and manpages

[mgorny]

Summary

Add a dependency between HTML & manpage Sphinx targets to prevent two instances of Sphinx from running in parallel, and therefore solves race conditions reusing the same doctree directory.

Bug: https://llvm.org/bugs/show_bug.cgi?id=23781

Also submitted for llvm: https://reviews.llvm.org/D23755

[axw]

LGTM

[mgorny]

This is no longer necessary since updated patch to D23755 handles all the projects using the functions.

docs/CMakeLists.txt

if (${SPHINX_OUTPUT_HTML})
       add_sphinx_target(html clang)
       add_custom_command(TARGET docs-clang-html POST_BUILD
         COMMAND ${CMAKE_COMMAND} -E copy
         "${CMAKE_CURRENT_SOURCE_DIR}/LibASTMatchersReference.html"
         "${CMAKE_CURRENT_BINARY_DIR}/html/LibASTMatchersReference.html")
     endif()
     if (${SPHINX_OUTPUT_MAN})
       add_sphinx_target(man clang)
+      if (${SPHINX_OUTPUT_HTML})
+        add_dependencies(docs-clang-html docs-clang-man)
+      endif()
     endif()
   endif()
 endif()