diff --git a/Makefile b/Makefile
index 89e04fb0c0..e96f1fb4cd 100644
--- a/Makefile
+++ b/Makefile
@@ -27,7 +27,7 @@ ifeq ($(Q),@)
 MAKEFLAGS += --no-print-directory
 endif
 
-PAPARAZZI_SRC=$(shell pwd)
+PAPARAZZI_SRC ?= $(shell pwd)
 empty=
 space=$(empty) $(empty)
 ifneq ($(findstring $(space),$(PAPARAZZI_SRC)),)
diff --git a/Makefile.ac b/Makefile.ac
index 55a30eef1b..d85e2bbd68 100644
--- a/Makefile.ac
+++ b/Makefile.ac
@@ -21,7 +21,7 @@
 
 # Preprocessing of XML configuration files
 
-PAPARAZZI_SRC=$(shell pwd)
+PAPARAZZI_SRC ?= $(shell pwd)
 
 include conf/Makefile.local