diff --git a/doc/Makefile b/doc/Makefile
index 311ce3ca..beadac7d 100644
--- a/doc/Makefile
+++ b/doc/Makefile
@@ -2,9 +2,9 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    =
-SPHINXBUILD   = sphinx-build
-PAPER         =
+SPHINXOPTS    ?=
+SPHINXBUILD   ?= sphinx-build
+PAPER         ?=
 
 # Internal variables.
 PAPEROPT_a4     = -D latex_paper_size=a4