[gtk-doc] Initial setup

Initial setup of gtk-doc. Straight forward setup following the gtk-doc
instructions. Ignore some troublesome types in src/hb-gobject.h. To
build use "./autogen.sh --enable-gtk-doc" then "make". Docs are in
harfbuzz/docs/reference/html/index.html.

src/Makefile.am

diff --git a/src/Makefile.am b/src/Makefile.am
index 73804a1..e646d4c 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -293,7 +293,7 @@
 	hb-ot-shape-complex-myanmar-machine.rl \
 	hb-ot-shape-complex-sea-machine.rl \
 	$(NULL)
-%.hh: %.rl
+.rl.hh:
 	$(AM_V_GEN)$(top_srcdir)/missing --run ragel -e -F1 -o "$@.tmp" "$<" && \
 	mv "$@.tmp" "$@" || ( $(RM) "$@.tmp" && false )
 

src/hb-gobject.h

diff --git a/src/hb-gobject.h b/src/hb-gobject.h
index 4f23fdd..3076460 100644
--- a/src/hb-gobject.h
+++ b/src/hb-gobject.h
@@ -33,7 +33,7 @@
 
 HB_BEGIN_DECLS
 
-
+#ifndef __GTK_DOC_IGNORE__
 /* Objects */
 
 #define HB_GOBJECT_TYPE_BLOB hb_gobject_blob_get_type ()
@@ -60,6 +60,7 @@
 GType
 hb_gobject_unicode_funcs_get_type (void);
 
+#endif /* __GTK_DOC_IGNORE__ */
 
 /* Enums */