fseek: avoid compilation failure when fflush is replaced

fseek: avoid compilation failure when fflush is replaced

./gnulib-tool --import fseek fwritable failed on Debian, due
to broken fflush() pulling in rpl_fseeko that mistakenly
overwrote fseek() as function-like macro.

* m4/fseek.m4 (gl_REPLACE_FSEEK): New macro.
* m4/fseeko.m4 (gl_REPLACE_FSEEKO): Also replace fseek, if fseek
module is in use.
* lib/stdio.in.h (GNULIB_FSEEKO): Only poison fseek if fseek
module is not in use; since REPLACE_FSEEK worked otherwise.
(GNULIB_FTELLO): Likewise for ftell.
Reported by Ian Beckwith and others.

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