stdio: don't assume gets any more

stdio: don't assume gets any more

Gnulib intentionally does not have a gets module, and now that C11
and glibc have dropped it, we should be more proactive about warning
any user on a platform that still has a declaration of this dangerous
interface.

* m4/stdio_h.m4 (gl_STDIO_H, gl_STDIO_H_DEFAULTS): Drop gets
support.
* modules/stdio (Makefile.am): Likewise.
* lib/stdio-read.c (gets): Likewise.
* tests/test-stdio-c++.cc: Likewise.
* m4/warn-on-use.m4 (gl_WARN_ON_USE_PREPARE): Fix comment.
* lib/stdio.in.h (gets): Make warning occur in more places.
* doc/posix-functions/gets.texi (gets): Update documentation.
Reported by Christer Solskogen.

Signed-off-by: Eric Blake <eblake@redhat.com>