wcpcpy, wcpncpy: Ensure declaration on glibc >= 2.13 systems.

wcpcpy, wcpncpy: Ensure declaration on glibc >= 2.13 systems.

* m4/wcpcpy.m4 (gl_FUNC_WCPCPY): Require AC_USE_SYSTEM_EXTENSIONS.
* m4/wcpncpy.m4 (gl_FUNC_WCPNCPY): Likewise.
* modules/wcpcpy (Depends-on): Add extensions.
* modules/wcpncpy (Depends-on): Likewise.
* doc/posix-functions/wcpcpy.texi: Mention missing declaration on glibc
systems.
* doc/posix-functions/wcpncpy.texi: Likewise.
* doc/posix-functions/wcwidth.texi: Likewise.
(cherry picked from commit b64c50cfe4fca783f23341fffebc7d8f84f58820)