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/diff.h

@@ -1,33 +1,8 @@
 #ifndef NVIM_DIFF_H
 #define NVIM_DIFF_H
 
-void diff_buf_delete(buf_T *buf);
-void diff_buf_adjust(win_T *win);
-void diff_buf_add(buf_T *buf);
-void diff_invalidate(buf_T *buf);
-void diff_mark_adjust(linenr_T line1, linenr_T line2, long amount,
-                      long amount_after);
-void ex_diffupdate(exarg_T *eap);
-void ex_diffpatch(exarg_T *eap);
-void ex_diffsplit(exarg_T *eap);
-void ex_diffthis(exarg_T *eap);
-void diff_win_options(win_T *wp, int addbuf);
-void ex_diffoff(exarg_T *eap);
-void diff_clear(tabpage_T *tp);
-int diff_check(win_T *wp, linenr_T lnum);
-int diff_check_fill(win_T *wp, linenr_T lnum);
-void diff_set_topline(win_T *fromwin, win_T *towin);
-int diffopt_changed(void);
-int diffopt_horizontal(void);
-int diff_find_change(win_T *wp, linenr_T lnum, int *startp, int *endp);
-int diff_infold(win_T *wp, linenr_T lnum);
-void nv_diffgetput(int put);
-void ex_diffgetput(exarg_T *eap);
-int diff_mode_buf(buf_T *buf);
-int diff_move_to(int dir, long count);
-linenr_T diff_get_corresponding_line(buf_T *buf1, linenr_T lnum1,
-                                     buf_T *buf2,
-                                     linenr_T lnum3);
-linenr_T diff_lnum_win(linenr_T lnum, win_T *wp);
 
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "diff.h.generated.h"
+#endif
 #endif  // NVIM_DIFF_H