[CMake] Allow building Sphinx docs in nit-picky mode

[EricWF]

Summary

The actual logic to build http://llvm.org/libcxx/docs does so with "-W" and "-n" enabled, meaning that the website will not be updated if there are any missing references.
Currently there is no way to emulate this behavior in CMake.

This patch adds an option to enable Sphinx's nit-picky mode so that the libc++ Sphinx buildbot can test with this option enabled.

[EricWF]

Use spaces not tabs.

[EricWF]

Abandoning. This option is only needed when using old versions of Sphinx.

cmake/modules/AddSphinxTarget.cmake

 # Handy function for creating the different Sphinx targets.
 #
 # ``builder`` should be one of the supported builders used by
 # the sphinx-build command.
 #
 # ``project`` should be the project name
 function (add_sphinx_target builder project)
   set(SPHINX_BUILD_DIR "${CMAKE_CURRENT_BINARY_DIR}/${builder}")
   set(SPHINX_DOC_TREE_DIR "${CMAKE_CURRENT_BINARY_DIR}/_doctrees")
   set(SPHINX_TARGET_NAME docs-${project}-${builder})
 
   if (SPHINX_WARNINGS_AS_ERRORS)
     set(SPHINX_WARNINGS_AS_ERRORS_FLAG "-W")
   else()
     set(SPHINX_WARNINGS_AS_ERRORS_FLAG "")
   endif()
+  if (SPHINX_WARN_MISSING_REFERENCES)
+    set(SPHINX_WARN_MISSING_REFERENCES_FLAG "-n")
+  else()
+    set(SPHINX_WARN_MISSING_REFERENCES_FLAG "")
+  endif()
 
   add_custom_target(${SPHINX_TARGET_NAME}
                     COMMAND ${SPHINX_EXECUTABLE}
                             -b ${builder}
                             -d "${SPHINX_DOC_TREE_DIR}"
                             -q # Quiet: no output other than errors and warnings.
                             ${SPHINX_WARNINGS_AS_ERRORS_FLAG} # Treat warnings as errors if requested
+                            ${SPHINX_WARN_MISSING_REFERENCES_FLAG} # Treat missing references as warnings
                             "${CMAKE_CURRENT_SOURCE_DIR}" # Source
                             "${SPHINX_BUILD_DIR}" # Output
                     COMMENT
                     "Generating ${builder} Sphinx documentation for ${project} into \"${SPHINX_BUILD_DIR}\"")
 
   # When "clean" target is run, remove the Sphinx build directory
   set_property(DIRECTORY APPEND PROPERTY
                ADDITIONAL_MAKE_CLEAN_FILES