fpurge: migrate into <stdio.h>

fpurge: migrate into <stdio.h>

* lib/fpurge.h: Delete...
* lib/stdio.in.h (fpurge): ...and declare here, instead.
* lib/fpurge.c (fpurge): Change declaring header.
* modules/fpurge (Files): Drop deleted file.
(Depends-on): Add stdio.
(configure.ac): Set witness.
* modules/stdio (Makefile.am): Support fpurge macros.
* m4/stdio_h.m4 (gl_STDIO_H_DEFAULTS): Likewise.
* m4/fpurge.m4 (gl_FUNC_FPURGE): Set appropriate variables.
* lib/fflush.c: Update client.
* tests/test-fpurge.c: Likewise.
* NEWS: Mention the change.

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