Index: src/Makefile.sub.in
--- src/Makefile.sub.in.orig
+++ src/Makefile.sub.in
@@ -5,7 +5,8 @@ SHELL=@SHELL@
 
 CC=@CC@
 LD=@LD@
-CFLAGS=@CFLAGS@ -I..
+CFLAGS=@CFLAGS@
+CPPFLAGS=@CPPFLAGS@ -I..
 OBJEXT=@OBJEXT@
 
 ifeq '$(SOURCES)' ''
@@ -35,10 +36,10 @@ endif
 
 .PHONY: depend
 depend:
-	$(CC) $(CFLAGS) -MM $(SOURCES) >.dependencies
+	$(CC) $(CPPFLAGS) -MM $(SOURCES) >.dependencies
 
 .dependencies: $(SOURCES) $(HEADERS)
-	$(CC) $(CFLAGS) -MM $(SOURCES) >$@
+	$(CC) $(CPPFLAGS) -MM $(SOURCES) >$@
 
 .PHONY: clean
 clean: