diff --git a/doc/developer/Makefile b/doc/developer/Makefile
index 640a1ace38..f5e7cb60ec 100644
--- a/doc/developer/Makefile
+++ b/doc/developer/Makefile
@@ -2,9 +2,9 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    =
-SPHINXBUILD   = sphinx-build
-PAPER         =
+SPHINXOPTS    ?=
+SPHINXBUILD   ?= sphinx-build
+PAPER         ?=
 BUILDDIR      = _build
 
 # User-friendly check for sphinx-build
diff --git a/doc/user/Makefile b/doc/user/Makefile
index 640a1ace38..f5e7cb60ec 100644
--- a/doc/user/Makefile
+++ b/doc/user/Makefile
@@ -2,9 +2,9 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    =
-SPHINXBUILD   = sphinx-build
-PAPER         =
+SPHINXOPTS    ?=
+SPHINXBUILD   ?= sphinx-build
+PAPER         ?=
 BUILDDIR      = _build
 
 # User-friendly check for sphinx-build