Warn if no Unicode functions implementation is found
diff --git a/src/hb-unicode-private.hh b/src/hb-unicode-private.hh
index c06dfe5..519fa0b 100644
--- a/src/hb-unicode-private.hh
+++ b/src/hb-unicode-private.hh
@@ -97,6 +97,7 @@
 extern HB_INTERNAL hb_unicode_funcs_t _hb_icu_unicode_funcs;
 #define _hb_unicode_funcs_default _hb_icu_unicode_funcs
 #else
+#define HB_UNICODE_FUNCS_NIL 1
 extern HB_INTERNAL hb_unicode_funcs_t _hb_unicode_funcs_nil;
 #define _hb_unicode_funcs_default _hb_unicode_funcs_nil
 #endif