Fix building with Sphinx 1.3
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 PYTHON        = python3
-SPHINXOPTS    = -W
+SPHINXOPTS    =
 SPHINXBUILD   = $(PYTHON) -m sphinx
 DOCDIR        = doc
 DOCHTML       = $(DOCDIR)/html