Dont define an empty SPHINXOPTS, which overrides a definition from the environment.

birkenfeld · birkenfeld · commit bb675280d91e · 2014-09-30T22:23:57.000+02:00
diff --git a/Doc/Makefile b/Doc/Makefile
@@ -6,7 +6,6 @@
 # You can set these variables from the command line.
 PYTHON       = python
 SPHINXBUILD  = sphinx-build
-SPHINXOPTS   =
 PAPER        =
 SOURCES      =
 DISTVERSION  = $(shell $(PYTHON) tools/sphinxext/patchlevel.py)
