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/option.c

@@ -1838,59 +1838,9 @@ static char *(p_fdm_values[]) = {"manual", "expr", "marker", "indent", "syntax",
 static char *(p_fcl_values[]) = {"all", NULL};
 static char *(p_cot_values[]) = {"menu", "menuone", "longest", "preview", NULL};
 
-static void set_option_default(int, int opt_flags, int compatible);
-static void set_options_default(int opt_flags);
-static char_u *term_bg_default(void);
-static void did_set_option(int opt_idx, int opt_flags, int new_value);
-static char_u *illegal_char(char_u *, int);
-static int string_to_key(char_u *arg);
-static char_u *check_cedit(void);
-static void did_set_title(int icon);
-static char_u *option_expand(int opt_idx, char_u *val);
-static void didset_options(void);
-static void check_string_option(char_u **pp);
-static long_u *insecure_flag(int opt_idx, int opt_flags);
-static void set_string_option_global(int opt_idx, char_u **varp);
-static char_u *set_string_option(int opt_idx, char_u *value,
-                                 int opt_flags);
-static char_u *did_set_string_option(int opt_idx, char_u **varp,
-                                     int new_value_alloced,
-                                     char_u *oldval, char_u *errbuf,
-                                     int opt_flags);
-static char_u *set_chars_option(char_u **varp);
-static int int_cmp(const void *a, const void *b);
-static char_u *compile_cap_prog(synblock_T *synblock);
-static void set_option_scriptID_idx(int opt_idx, int opt_flags, int id);
-static char_u *set_bool_option(int opt_idx, char_u *varp, int value,
-                               int opt_flags);
-static char_u *set_num_option(int opt_idx, char_u *varp, long value,
-                              char_u *errbuf, size_t errbuflen,
-                              int opt_flags);
-static void check_redraw(long_u flags);
-static int findoption(char_u *);
-static int find_key_option(char_u *);
-static void showoptions(int all, int opt_flags);
-static int optval_default(struct vimoption *, char_u *varp);
-static void showoneopt(struct vimoption *, int opt_flags);
-static int put_setstring(FILE *fd, char *cmd, char *name, char_u **valuep,
-                         int expand);
-static int put_setnum(FILE *fd, char *cmd, char *name, long *valuep);
-static int put_setbool(FILE *fd, char *cmd, char *name, int value);
-static int istermoption(struct vimoption *);
-static char_u *get_varp_scope(struct vimoption *p, int opt_flags);
-static char_u *get_varp(struct vimoption *);
-static void option_value2string(struct vimoption *, int opt_flags);
-static int wc_use_keyname(char_u *varp, long *wcp);
-static void langmap_init(void);
-static void langmap_set(void);
-static void paste_option_changed(void);
-static void compatible_set(void);
-static void fill_breakat_flags(void);
-static int opt_strings_flags(char_u *val, char **values,
-                             unsigned *flagp,
-                             int list);
-static int check_opt_strings(char_u *val, char **values, int);
-static int check_opt_wim(void);
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "option.c.generated.h"
+#endif
 
 /*
  * Initialize the options, first part.
@@ -3688,7 +3638,6 @@ static long_u *insecure_flag(int opt_idx, int opt_flags)
   return &options[opt_idx].flags;
 }
 
-static void redraw_titles(void);
 
 /*
  * Redraw the window title and/or tab page text later.
@@ -7572,7 +7521,6 @@ typedef struct {
 } langmap_entry_T;
 
 static garray_T langmap_mapga;
-static void langmap_set_entry(int from, int to);
 
 /*
  * Search for an entry in "langmap_mapga" for "from".  If found set the "to"