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/os_unix.h

@@ -3,44 +3,7 @@
 
 #include "nvim/os/shell.h"
 
-/* os_unix.c */
-void mch_write(char_u *s, int len);
-void mch_suspend(void);
-void mch_init(void);
-int mch_can_restore_title(void);
-int mch_can_restore_icon(void);
-void mch_settitle(char_u *title, char_u *icon);
-void mch_restore_title(int which);
-int vim_is_xterm(char_u *name);
-int use_xterm_like_mouse(char_u *name);
-int use_xterm_mouse(void);
-int vim_is_iris(char_u *name);
-int vim_is_vt300(char_u *name);
-int vim_is_fastterm(char_u *name);
-void slash_adjust(char_u *p);
-void fname_case(char_u *name, int len);
-void mch_copy_sec(char_u *from_file, char_u *to_file);
-vim_acl_T mch_get_acl(char_u *fname);
-void mch_set_acl(char_u *fname, vim_acl_T aclent);
-void mch_free_acl(vim_acl_T aclent);
-void mch_hide(char_u *name);
-int mch_nodetype(char_u *name);
-void mch_early_init(void);
-void mch_free_mem(void);
-void mch_exit(int r);
-void mch_settmode(int tmode);
-void get_stty(void);
-void mch_setmouse(int on);
-void check_mouse_termcode(void);
-int mch_get_shellsize(void);
-void mch_set_shellsize(void);
-int mch_expand_wildcards(int num_pat, char_u **pat, int *num_file,
-                         char_u ***file,
-                         int flags);
-int mch_has_exp_wildcard(char_u *p);
-int mch_has_wildcard(char_u *p);
-int mch_libcall(char_u *libname, char_u *funcname, char_u *argstring,
-                int argint, char_u **string_result,
-                int *number_result);
-
-#endif /* NVIM_OS_UNIX_H */
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "os_unix.h.generated.h"
+#endif
+#endif  // NVIM_OS_UNIX_H