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/menu.h

@@ -48,18 +48,8 @@ struct VimMenu {
   vimmenu_T   *next;                /* Next item in menu */
 };
 
-void ex_menu(exarg_T *eap);
-char_u *set_context_in_menu_cmd(expand_T *xp, char_u *cmd, char_u *arg,
-                                int forceit);
-char_u *get_menu_name(expand_T *xp, int idx);
-char_u *get_menu_names(expand_T *xp, int idx);
-char_u *menu_name_skip(char_u *name);
-int menu_is_menubar(char_u *name);
-int menu_is_popup(char_u *name);
-int menu_is_toolbar(char_u *name);
-int menu_is_separator(char_u *name);
-void ex_emenu(exarg_T *eap);
-vimmenu_T *gui_find_menu(char_u *path_name);
-void ex_menutranslate(exarg_T *eap);
 
-#endif /* NVIM_MENU_H */
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "menu.h.generated.h"
+#endif
+#endif  // NVIM_MENU_H