* m4/printf-posix.m4: Renamed from m4/printf-posix-rpl.m4.
(gl_FUNC_PRINTF_IS_POSIX): New macro.
(gl_FUNC_PRINTF_POSIX): Require it. Don't require
gl_FUNC_VFPRINTF_POSIX. Invoke gl_PREREQ_VASNPRINTF_WITH_POSIX_EXTRAS
and gl_REPLACE_VASNPRINTF.
* m4/printf-gnu.m4: New file, based on m4/fprintf-posix.m4.
* modules/printf-posix (Files): Update.
* modules/printf-gnu: New file, based on modules/vasnprintf-gnu.
* doc/posix-functions/printf.texi: Mention the new module.