[config] Add HB_LEAN

Part of https://github.com/harfbuzz/harfbuzz/issues/1652
diff --git a/src/hb-config.hh b/src/hb-config.hh
index 6c7a4eb..b13215a 100644
--- a/src/hb-config.hh
+++ b/src/hb-config.hh
@@ -36,9 +36,14 @@
 
 
 #ifdef HB_TINY
+#define HB_LEAN
 #define HB_MINI
+#endif
+
+#ifdef HB_LEAN
 #define HB_NO_BITMAP
 #define HB_NO_CFF
+#define HB_NO_MATH
 #endif
 
 #ifdef HB_MINI