Remove support for systems without ftello/fseeko
fseeko and ftello conforms to SUSv2, POSIX.1-2001. The configure.in says:
# BSDI doesn't have ftello fseeko
AC_CHECK_FUNCS(ftello fseeko)The last version of BSD/OS was released in 2003, henceforth I believe it's safe to remove this workaround.