whether fseeko and ftello are properly declared.
* acfunctions.m4 (AC_FUNC_FSEEKO): New macro, which worries about
fseeko (and presumably ftello). Do not set _XOPEN_SOURCE; that
causes too many problems in practice.
* acfunctions (fteelo, fseeko): Trigger AC_FUNC_FSEEKO.
* doc/autoconf.texi: Adjust.