--- work/siputils.py.orig	2017-09-04 07:49:58.370772959 +0000
+++ work/siputils.py	2017-09-04 07:50:07.367771132 +0000
@@ -1157,9 +1157,9 @@
         if self._deployment_target:
             mfile.write("export MACOSX_DEPLOYMENT_TARGET = %s\n" % self._deployment_target)
 
-        mfile.write("CC = %s\n" % self.required_string("CC"))
-        mfile.write("CXX = %s\n" % self.required_string("CXX"))
-        mfile.write("LINK = %s\n" % self.required_string("LINK"))
+        mfile.write("CC ?= %s\n" % self.required_string("CC"))
+        mfile.write("CXX ?= %s\n" % self.required_string("CXX"))
+        mfile.write("LINK ?= %s\n" % self.required_string("LINK"))
 
         cppflags = []