* m4/vprintf-posix.m4 (gl_FUNC_VPRINTF_IS_POSIX): New macro.
(gl_FUNC_VPRINTF_POSIX): Require it. Don't require
gl_FUNC_VFPRINTF_POSIX. Invoke gl_PREREQ_VASNPRINTF_WITH_POSIX_EXTRAS
and gl_REPLACE_VASNPRINTF.
* m4/vprintf-gnu.m4: New file, based on m4/vfprintf-posix.m4.
* modules/vprintf-gnu: New file, based on modules/vasnprintf-gnu.
* doc/posix-functions/vprintf.texi: Mention the new module.
