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/main.h

@@ -3,14 +3,7 @@
 
 #include "nvim/normal.h"
 
-void main_loop(int cmdwin, int noexmode);
-void getout(int exitval);
-int process_env(char_u *env, int is_viminit);
-void mainerr_arg_missing(char_u *str);
-void time_push(void *tv_rel, void *tv_start);
-void time_pop(void *tp);
-void time_msg(char *mesg, void *tv_start);
-char_u *eval_client_expr_to_string(char_u *expr);
-char_u *serverConvert(char_u *client_enc, char_u *data, char_u **tofree);
-
-#endif /* NVIM_MAIN_H */
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "main.h.generated.h"
+#endif
+#endif  // NVIM_MAIN_H