diff --git a/docs/src/Makefile b/docs/src/Makefile
index 2ecf7a32..ba87d170 100644
--- a/docs/src/Makefile
+++ b/docs/src/Makefile
@@ -2,10 +2,10 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    =
-SPHINXBUILD   = sphinx-build
-PAPER         =
-BUILDDIR      = ../dist
+SPHINXOPTS   ?=
+SPHINXBUILD  ?= sphinx-build
+PAPER        ?=
+BUILDDIR     ?= ../dist
 # for cache, etc.
 _BUILDDIR     = _build