* m4/obstack-printf-posix.m4 (gl_FUNC_OBSTACK_PRINTF_IS_POSIX): New
macro, extracted from gl_FUNC_OBSTACK_PRINTF_POSIX.
(gl_FUNC_OBSTACK_PRINTF_POSIX): Require it. Invoke
gl_PREREQ_VASNPRINTF_WITH_POSIX_EXTRAS.
* m4/obstack-printf-gnu.m4: New file, based on
m4/obstack-printf-posix.m4.
* modules/obstack-printf-gnu: New file, based on modules/vasnprintf-gnu.
* doc/glibc-functions/obstack_printf.texi: Mention the new module.
* doc/glibc-functions/obstack_vprintf.texi: Likewise.