Allowed overriding variables in docs/Makefile.

docs/Makefile

@@ -2,11 +2,11 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    =
-SPHINXBUILD   = sphinx-build
-PAPER         =
-BUILDDIR      = _build
-LANGUAGE      =
+SPHINXOPTS    ?=
+SPHINXBUILD   ?= sphinx-build
+PAPER         ?=
+BUILDDIR      ?= _build
+LANGUAGE      ?=
 
 # Internal variables.
 PAPEROPT_a4     = -D latex_paper_size=a4