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/quickfix.h

@@ -1,36 +1,8 @@
 #ifndef NVIM_QUICKFIX_H
 #define NVIM_QUICKFIX_H
 /* quickfix.c */
-int qf_init(win_T *wp, char_u *efile, char_u *errorformat, int newlist,
-            char_u *qf_title);
-void qf_free_all(win_T *wp);
-void copy_loclist(win_T *from, win_T *to);
-void qf_jump(qf_info_T *qi, int dir, int errornr, int forceit);
-void qf_list(exarg_T *eap);
-void qf_age(exarg_T *eap);
-void qf_mark_adjust(win_T *wp, linenr_T line1, linenr_T line2,
-                    long amount,
-                    long amount_after);
-void ex_cwindow(exarg_T *eap);
-void ex_cclose(exarg_T *eap);
-void ex_copen(exarg_T *eap);
-linenr_T qf_current_entry(win_T *wp);
-int bt_quickfix(buf_T *buf);
-int bt_nofile(buf_T *buf);
-int bt_dontwrite(buf_T *buf);
-int bt_dontwrite_msg(buf_T *buf);
-int buf_hide(buf_T *buf);
-int grep_internal(cmdidx_T cmdidx);
-void ex_make(exarg_T *eap);
-void ex_cc(exarg_T *eap);
-void ex_cnext(exarg_T *eap);
-void ex_cfile(exarg_T *eap);
-void ex_vimgrep(exarg_T *eap);
-char_u *skip_vimgrep_pat(char_u *p, char_u **s, int *flags);
-int get_errorlist(win_T *wp, list_T *list);
-int set_errorlist(win_T *wp, list_T *list, int action, char_u *title);
-void ex_cbuffer(exarg_T *eap);
-void ex_cexpr(exarg_T *eap);
-void ex_helpgrep(exarg_T *eap);
 
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "quickfix.h.generated.h"
+#endif
 #endif /* NVIM_QUICKFIX_H */