* m4/vdprintf-posix.m4 (gl_FUNC_VDPRINTF_IS_POSIX): New macro,
extracted from gl_FUNC_VDPRINTF_POSIX.
(gl_FUNC_VDPRINTF_POSIX): Require it. Invoke
gl_PREREQ_VASNPRINTF_WITH_POSIX_EXTRAS.
* m4/vdprintf-gnu.m4: New file, based on m4/vdprintf-posix.m4.
* modules/vdprintf-gnu: New file, based on modules/vasnprintf-gnu.
* doc/posix-functions/vdprintf.texi: Mention the new module.
