obstack-printf-posix: ensure declaration

obstack-printf-posix: ensure declaration

The replacement <stdio.h> was failing to declare obstack_printf
if obstack-printf-posix was used in isolation without unit tests.

* m4/obstack-printf.m4 (gl_DECL_OBSTACK_PRINTF): New macro,
extracted from gl_FUNC_OBSTACK_PRINTF.
(gl_FUNC_OBSTACK_PRINTF): Invoke it.
* m4/obstack-printf-posix.m4 (gl_FUNC_OBSTACK_PRINTF_POSIX):
Likewise.
* lib/stdio.in.h (obstack_printf, obstack_vprintf): Declare also
if GNULIB_OBSTACK_PRINTF_POSIX is 1 and GNULIB_OBSTACK_PRINTF is
0.

Signed-off-by: Eric Blake <ebb9@byu.net>