Import UCDN into source tree

https://github.com/grigorig/ucdn
diff --git a/src/Makefile.am b/src/Makefile.am
index d91113a..9f7c7f8 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -178,6 +178,13 @@
 endif
 DIST_SUBDIRS += hb-icu-le
 
+if HAVE_UCDN
+SUBDIRS += hb-ucdn
+HBCFLAGS += -I$(srcdir)/hb-ucdn
+HBLIBS   += hb-ucdn/libhb-ucdn.la
+HBSOURCES += hb-ucdn.cc
+endif
+DIST_SUBDIRS += hb-ucdn
 
 
 # Put the library together