Allow disabling multi-threaded support

By defining HB_NO_MT.

Also, only warn once per missing MT feature support.

Mozilla Bug 666661 - gfx/harfbuzz/src/hb-prive.h - compiler warnings on mac
diff --git a/src/Makefile.am b/src/Makefile.am
index 8e8c013..d747895 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -38,6 +38,7 @@
 	hb-tt-font.cc \
 	hb-unicode-private.hh \
 	hb-unicode.cc \
+	hb-warning.cc \
 	$(NULL)
 HBHEADERS = \
 	hb.h \