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/hardcopy.h

@@ -70,24 +70,8 @@ typedef struct {
 
 #define PRINT_NUMBER_WIDTH 8
 
-char_u *parse_printoptions(void);
-char_u *parse_printmbfont(void);
-int prt_header_height(void);
-int prt_use_number(void);
-int prt_get_unit(int idx);
-void ex_hardcopy(exarg_T *eap);
-void mch_print_cleanup(void);
-int mch_print_init(prt_settings_T *psettings, char_u *jobname,
-                   int forceit);
-int mch_print_begin(prt_settings_T *psettings);
-void mch_print_end(prt_settings_T *psettings);
-int mch_print_end_page(void);
-int mch_print_begin_page(char_u *str);
-int mch_print_blank_page(void);
-void mch_print_start_line(int margin, int page_line);
-int mch_print_text_out(char_u *p, int len);
-void mch_print_set_font(int iBold, int iItalic, int iUnderline);
-void mch_print_set_bg(long_u bgcol);
-void mch_print_set_fg(long_u fgcol);
 
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "hardcopy.h.generated.h"
+#endif
 #endif /* NVIM_HARDCOPY_H */