--- a/Makefile.rules
+++ b/Makefile.rules
@@ -506,11 +506,13 @@ endif
 #--------------------------------------------------------------------
 
 ifeq ($(HOST_OS),Darwin)
+  ifndef DARWIN_VERSION
   DARWIN_VERSION := `sw_vers -productVersion`
   # Strip a number like 10.4.7 to 10.4
   DARWIN_VERSION := $(shell echo $(DARWIN_VERSION)| sed -E 's/(10.[0-9]).*/\1/')
   # Get "4" out of 10.4 for later pieces in the makefile.
   DARWIN_MAJVERS := $(shell echo $(DARWIN_VERSION)| sed -E 's/10.([0-9]).*/\1/')
+  endif
 
   SharedLinkOptions=-Wl,-flat_namespace -Wl,-undefined -Wl,suppress \
                     -dynamiclib