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/keymap.h

@@ -498,18 +498,8 @@ enum key_extra {
  */
 #define MAX_KEY_CODE_LEN    6
 
-int name_to_mod_mask(int c);
-int simplify_key(int key, int *modifiers);
-int handle_x_keys(int key);
-char_u *get_special_key_name(int c, int modifiers);
-int trans_special(char_u **srcp, char_u *dst, int keycode);
-int find_special_key(char_u **srcp, int *modp, int keycode,
-                     int keep_x_key);
-int extract_modifiers(int key, int *modp);
-int find_special_key_in_table(int c);
-int get_special_key_code(char_u *name);
-char_u *get_key_name(int i);
-int get_mouse_button(int code, int *is_click, int *is_drag);
-int get_pseudo_mouse_code(int button, int is_click, int is_drag);
 
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "keymap.h.generated.h"
+#endif
 #endif /* NVIM_KEYMAP_H */