* lib/string.in.h, lib/wchar.in.h:
(free): Declare by hand instead of including stdlib.h.
This avoids some namespace pollution. It should also avoid some
nested-include problems described by Lucy Phipps in:
https://lists.gnu.org/r/bug-gnulib/2021-09/msg00018.html
* modules/string, modules/wchar:
(Depends-on): Add stdlib, so that REPLACE_FREE has the right value.
(Makefile.am): Replace @REPLACE_FREE@ when creating the include file.