diff --git a/Makefile b/Makefile
index 7eaf454..fa4f38b 100644
--- a/Makefile
+++ b/Makefile
@@ -318,8 +318,8 @@
 #
 ifeq ($(HOSTOS),darwin)
 # get major and minor product version (e.g. '10' and '6' for Snow Leopard)
-DARWIN_MAJOR_VERSION	= $(shell sw_vers -productVersion | cut -f 1 -d '.')
-DARWIN_MINOR_VERSION	= $(shell sw_vers -productVersion | cut -f 2 -d '.')
+DARWIN_MAJOR_VERSION	:= $(shell sw_vers -productVersion | cut -f 1 -d '.')
+DARWIN_MINOR_VERSION	:= $(shell sw_vers -productVersion | cut -f 2 -d '.')
 
 os_x_before	= $(shell if [ $(DARWIN_MAJOR_VERSION) -le $(1) -a \
 	$(DARWIN_MINOR_VERSION) -le $(2) ] ; then echo "$(3)"; else echo "$(4)"; fi ;)