diff --git a/Makefile b/Makefile
index 0d9de5aa8..67d312300 100644
--- a/Makefile
+++ b/Makefile
@@ -147,7 +147,7 @@ default: all
 ### Paths
 ############################################################
 
-PYTHONPATH = ${SOURCE_ROOT}/tools
+PYTHONPATH := ${PYTHONPATH}:${SOURCE_ROOT}/tools
 export PYTHONPATH
 
 vpath %.c   ${SOURCE_ROOT}