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.c

@@ -32,29 +32,13 @@
 
 #define MENUDEPTH   10          /* maximum depth of menus */
 
-static int add_menu_path(char_u *, vimmenu_T *, int *, char_u *);
-static int menu_nable_recurse(vimmenu_T *menu, char_u *name, int modes,
-                              int enable);
-static int remove_menu(vimmenu_T **, char_u *, int, int silent);
-static void free_menu(vimmenu_T **menup);
-static void free_menu_string(vimmenu_T *, int);
-static int show_menus(char_u *, int);
-static void show_menus_recursive(vimmenu_T *, int, int);
-static int menu_name_equal(char_u *name, vimmenu_T *menu);
-static int menu_namecmp(char_u *name, char_u *mname);
-static int get_menu_cmd_modes(char_u *, int, int *, int *);
-static char_u *popup_mode_name(char_u *name, int idx);
-static char_u *menu_text(char_u *text, int *mnemonic, char_u **actext);
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "menu.c.generated.h"
+#endif
 
 
-static int menu_is_hidden(char_u *name);
-static int menu_is_tearoff(char_u *name);
 
-static char_u *menu_skip_part(char_u *p);
-static char_u *menutrans_lookup(char_u *name, int len);
-static void menu_unescape_name(char_u  *p);
-
-static char_u *menu_translate_tab_and_shift(char_u *arg_start);
 
 /* The character for each menu mode */
 static char_u menu_mode_chars[] = {'n', 'v', 's', 'o', 'i', 'c', 't'};