Update Makefile

docs/Makefile

@@ -6,7 +6,7 @@ include ../config.sh
 
 # You can set these variables from the command line, and also
 # from the environment for the first two.
-SPHINXOPTS    +=
+SPHINXOPTS    ?=
 SPHINXBUILD   ?= $(PYTHON_VENV_MANAGER) $(PYTHON_VENV_MANAGER_RUN_CMD) sphinx-build
 SOURCEDIR     = source
 BUILDDIR      = build