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/cursor_shape.h

@@ -47,6 +47,8 @@ typedef struct cursor_entry {
   char used_for;                /* SHAPE_MOUSE and/or SHAPE_CURSOR */
 } cursorentry_T;
 
-char_u *parse_shape_opt(int what);
 
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "cursor_shape.h.generated.h"
+#endif
 #endif /* NVIM_CURSOR_SHAPE_H */