If HAVE_ICU_BUILTIN is defined, use hb-icu Unicode callbacks

Useful for Android / Chrome / etc when ICU is built in the same
library as harfbuzz itself.

src/hb-unicode.cc

diff --git a/src/hb-unicode.cc b/src/hb-unicode.cc
index 5b44913..ed99ca2 100644
--- a/src/hb-unicode.cc
+++ b/src/hb-unicode.cc
@@ -133,7 +133,7 @@
 
 #ifdef HAVE_GLIB
   HB_UNICODE_FUNCS_IMPLEMENT(glib)
-#elif 0 && defined(HAVE_ICU)
+#elif defined(HAVE_ICU) && defined(HAVE_ICU_BUILTIN)
   HB_UNICODE_FUNCS_IMPLEMENT(icu)
 #elif defined(HAVE_UCDN)
   HB_UNICODE_FUNCS_IMPLEMENT(ucdn)