Treat warnings in Sphinx as errors. The reasons for doing this are...

- When CMake builds the documentation with sphinx-build it treats
  warnings as errors. We should be consistent with what we do in
  CMake.
- Having warnings treated as errors will hopefully encourage
  developers to write documentation correctly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213661 91177308-0d34-0410-b5e6-96231b3b80d8

docs/Makefile.sphinx

@@ -10,7 +10,7 @@ BUILDDIR      = _build
 # Internal variables.
 PAPEROPT_a4     = -D latex_paper_size=a4
 PAPEROPT_letter = -D latex_paper_size=letter
-ALLSPHINXOPTS   = -d $(BUILDDIR)/doctrees $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
+ALLSPHINXOPTS   = -d $(BUILDDIR)/doctrees $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) -W .
 # the i18n builder cannot share the environment and doctrees with the others
 I18NSPHINXOPTS  = $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .