Make HB_PRIVATE more useful

So we can apply it to class methods also.  Not sure if that works!

src/hb-blob-private.h

@@ -50,7 +50,7 @@ struct _hb_blob_t {
   void *user_data;
 };
 
-HB_INTERNAL hb_blob_t _hb_blob_nil;
+extern HB_INTERNAL hb_blob_t _hb_blob_nil;
 
 HB_END_DECLS
 

src/hb-font-private.hh

@@ -50,8 +50,8 @@ struct _hb_font_funcs_t {
   hb_font_get_kerning_func_t		get_kerning;
 };
 
-HB_INTERNAL hb_font_funcs_t
-_hb_font_funcs_nil;
+extern HB_INTERNAL hb_font_funcs_t _hb_font_funcs_nil;
+
 
 /*
  * hb_face_t

src/hb-private.h

@@ -108,7 +108,7 @@
 #endif
 
 #ifndef HB_INTERNAL
-# define HB_INTERNAL extern __attribute__((__visibility__("hidden")))
+# define HB_INTERNAL __attribute__((__visibility__("hidden")))
 #endif
 
 

src/hb-unicode-private.h

@@ -49,8 +49,7 @@ struct _hb_unicode_funcs_t {
   hb_unicode_get_eastasian_width_func_t		get_eastasian_width;
 };
 
-HB_INTERNAL hb_unicode_funcs_t
-_hb_unicode_funcs_nil;
+extern HB_INTERNAL hb_unicode_funcs_t _hb_unicode_funcs_nil;
 
 
 HB_INTERNAL hb_direction_t