diagnostics: keep the fixits

Introduce proper support for fixits, instead of just printing them on
demand.

* bootstrap.conf: We need gnulib's xlists.
* src/fixits.h, src/fixits.c: New.
* src/complain.c (deprecated_directive): Use fixits_register.
* src/main.c (main): Use fixits_free.

src/fixits.h

@@ -0,0 +1,29 @@
+/* Support for fixing grammar files.
+
+   Copyright (C) 2019 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef FIXITS_H_
+# define FIXITS_H_ 1
+
+# include "location.h"
+
+/* Declare a fix to apply.  */
+void fixits_register (location const *loc, char const* update);
+
+/* Free the registered fixits.  */
+void fixits_free (void);
+
+#endif /* !FIXITS_H_ */