core: remove value for first #define in headers, add "PLUGIN" in plugin headers

src/plugins/alias/alias.h

@@ -17,8 +17,8 @@
  * along with WeeChat.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#ifndef WEECHAT_ALIAS_H
-#define WEECHAT_ALIAS_H 1
+#ifndef WEECHAT_PLUGIN_ALIAS_H
+#define WEECHAT_PLUGIN_ALIAS_H
 
 #define weechat_plugin weechat_alias_plugin
 #define ALIAS_PLUGIN_NAME "alias"
@@ -52,4 +52,4 @@ extern void alias_free_all ();
 extern int alias_add_to_infolist (struct t_infolist *infolist,
                                   struct t_alias *alias);
 
-#endif /* WEECHAT_ALIAS_H */
+#endif /* WEECHAT_PLUGIN_ALIAS_H */