diff --git a/siputils.py b/siputils.py
index e0efcf5..5da51f8 100644
--- a/siputils.py
+++ b/siputils.py
@@ -1162,9 +1162,9 @@ class Makefile:
         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 = []
 
@@ -1194,13 +1194,13 @@ class Makefile:
 
         libs.extend(self.optional_list("LIBS"))
 
-        mfile.write("CPPFLAGS = %s\n" % ' '.join(cppflags))
+        mfile.write("CPPFLAGS += %s\n" % ' '.join(cppflags))
 
-        mfile.write("CFLAGS = %s\n" % self.optional_string("CFLAGS"))
-        mfile.write("CXXFLAGS = %s\n" % self.optional_string("CXXFLAGS"))
-        mfile.write("LFLAGS = %s\n" % self.optional_string("LFLAGS"))
+        mfile.write("CFLAGS += %s\n" % self.optional_string("CFLAGS"))
+        mfile.write("CXXFLAGS += %s\n" % self.optional_string("CXXFLAGS"))
+        mfile.write("LFLAGS += %s\n" % self.optional_string("LFLAGS"))
 
-        mfile.write("LIBS = %s\n" % ' '.join(libs))
+        mfile.write("LIBS += %s\n" % ' '.join(libs))
 
         if self._qt:
             mfile.write("MOC = %s\n" % _quote(self.required_string("MOC")))