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/regexp.h

@@ -1,27 +1,8 @@
 #ifndef NVIM_REGEXP_H
 #define NVIM_REGEXP_H
 /* regexp.c */
-int re_multiline(regprog_T *prog);
-char_u *skip_regexp(char_u *startp, int dirc, int magic, char_u **newp);
-int vim_regcomp_had_eol(void);
-void free_regexp_stuff(void);
-reg_extmatch_T *ref_extmatch(reg_extmatch_T *em);
-void unref_extmatch(reg_extmatch_T *em);
-char_u *regtilde(char_u *source, int magic);
-int vim_regsub(regmatch_T *rmp, char_u *source, char_u *dest, int copy,
-                       int magic,
-                       int backslash);
-int vim_regsub_multi(regmmatch_T *rmp, linenr_T lnum, char_u *source,
-                             char_u *dest, int copy, int magic,
-                             int backslash);
-char_u *reg_submatch(int no);
-list_T *reg_submatch_list(int no);
-regprog_T *vim_regcomp(char_u *expr_arg, int re_flags);
-void vim_regfree(regprog_T *prog);
-int vim_regexec(regmatch_T *rmp, char_u *line, colnr_T col);
-int vim_regexec_nl(regmatch_T *rmp, char_u *line, colnr_T col);
-long vim_regexec_multi(regmmatch_T *rmp, win_T *win, buf_T *buf,
-                               linenr_T lnum, colnr_T col,
-                               proftime_T *tm);
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "regexp.h.generated.h"
+#endif
 
 #endif /* NVIM_REGEXP_H */