diff --git a/docs/Makefile b/docs/Makefile
index 3b412210..c686ed80 100644
--- a/docs/Makefile
+++ b/docs/Makefile
@@ -1,8 +1,8 @@
 # Makefile for Sphinx documentation
-SPHINXOPTS    =
-SPHINXBUILD   = sphinx-build
-PAPER         =
-BUILDDIR      = _build
+SPHINXOPTS    ?=
+SPHINXBUILD   ?= sphinx-build
+PAPER         ?=
+BUILDDIR      ?= _build
 
 # Internal variables
 PAPEROPT_a4     = -D latex_paper_size=a4