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/garray.h

@@ -18,16 +18,7 @@ typedef struct growarray {
 
 #define GA_EMPTY(ga_ptr) ((ga_ptr)->ga_len <= 0)
 
-void ga_clear(garray_T *gap);
-void ga_clear_strings(garray_T *gap);
-void ga_init(garray_T *gap, int itemsize, int growsize);
-void ga_grow(garray_T *gap, int n);
-char_u *ga_concat_strings_sep(const garray_T *gap, const char *sep)
-  FUNC_ATTR_NONNULL_RET;
-char_u *ga_concat_strings(const garray_T *gap) FUNC_ATTR_NONNULL_RET;
-void ga_remove_duplicate_strings(garray_T *gap);
-void ga_concat(garray_T *gap, const char_u *restrict s);
-void ga_append(garray_T *gap, char c);
-void append_ga_line(garray_T *gap);
-
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "garray.h.generated.h"
+#endif
 #endif  // NVIM_GARRAY_H