Make it an error to include non-top-level headers
Users should #include <hb.h> (or hb-ft.h, hb-glib.h, etc), but
never things like hb-shape.h directly. This makes it easier to
refactor headers later on without breaking compatibility.
diff --git a/src/hb.h b/src/hb.h
index 0a2ebd9..996dc91 100644
--- a/src/hb.h
+++ b/src/hb.h
@@ -26,6 +26,7 @@
#ifndef HB_H
#define HB_H
+#define HB_H_IN
#include "hb-blob.h"
#include "hb-buffer.h"
@@ -38,4 +39,5 @@
HB_BEGIN_DECLS
HB_END_DECLS
+#undef HB_H_IN
#endif /* HB_H */
