Add automatic generation of headers

- The 'stripdecls.py' script replaces declarations in all headers by includes to
  generated headers.
  `ag '#\s*if(?!ndef NEOVIM_).*((?!#\s*endif).*\n)*#ifdef INCLUDE_GENERATED'`
  was used for this.
- Add and integrate gendeclarations.lua into the build system to generate the
  required includes.
- Add -Wno-unused-function
- Made a bunch of old-style definitions ANSI

This adds a requirement: all type and structure definitions must be present
before INCLUDE_GENERATED_DECLARATIONS-protected include.

Warning: mch_expandpath (path.h.generated.h) was moved manually. So far it is
the only exception.

src/nvim/undo.h

@@ -3,34 +3,7 @@
 
 #include "nvim/undo_defs.h"
 
-/* undo.c */
-int u_save_cursor(void);
-int u_save(linenr_T top, linenr_T bot);
-int u_savesub(linenr_T lnum);
-int u_inssub(linenr_T lnum);
-int u_savedel(linenr_T lnum, long nlines);
-int undo_allowed(void);
-int u_savecommon(linenr_T top, linenr_T bot, linenr_T newbot,
-                 int reload);
-void u_compute_hash(char_u *hash);
-char_u *u_get_undo_file_name(char_u *buf_ffname, int reading);
-void u_write_undo(char_u *name, int forceit, buf_T *buf, char_u *hash);
-void u_read_undo(char_u *name, char_u *hash, char_u *orig_name);
-void u_undo(int count);
-void u_redo(int count);
-void undo_time(long step, int sec, int file, int absolute);
-void u_sync(int force);
-void ex_undolist(exarg_T *eap);
-void ex_undojoin(exarg_T *eap);
-void u_unchanged(buf_T *buf);
-void u_find_first_changed(void);
-void u_update_save_nr(buf_T *buf);
-void u_clearall(buf_T *buf);
-void u_saveline(linenr_T lnum);
-void u_clearline(void);
-void u_undoline(void);
-void u_blockfree(buf_T *buf);
-int bufIsChanged(buf_T *buf);
-int curbufIsChanged(void);
-void u_eval_tree(u_header_T *first_uhp, list_T *list);
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "undo.h.generated.h"
+#endif
 #endif /* NVIM_UNDO_H */