diff --git a/doc/Makefile.am b/doc/Makefile.am
index 13317991..ce56b1c2 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -272,7 +272,7 @@ EXTRA_DIST = \
 
 # You can set these variables from the command line.
 SPHINXOPTS    =
-SPHINXBUILD   = sphinx-build
+SPHINXBUILD   ?= sphinx-build
 PAPER         =
 BUILDDIR      = manual