--- Makefile.in.orig	2016-05-25 09:58:50 UTC
+++ Makefile.in
@@ -1,4 +1,5 @@
 CFLAGS = @CFLAGS@
+CPPFLAGS= @CPPFLAGS@
 LDFLAGS = @LDFLAGS@
 LIBS = @LIBS@
 CC = @CC@
@@ -57,4 +58,4 @@ install_html: $(EXE).html
 	$(INSTALL) -D -m 0644 $< $(DESTDIR)$(htmldir)/$<
 
 .c.o: $(HEADERS)
-	$(CC) -c $(CFLAGS) -DVERSION='"$(VERSION)"' $<
+	$(CC) -c $(CFLAGS) $(CPPFLAGS) -DVERSION='"$(VERSION)"' $<