[PATCH] fix AC_STAT64 for Linux

[Roland McGrath]

> Could you use __linux__ instead?  I had to go digging to check that
> LINUX was defined by this point (it is), but this is a property of the
> system and not of our configuration.

No, it's a property of our configure script.  AC_DEFINE_UNQUOTED($OPSYS) is
done before all the specific checks.  Autoconf will put all such
definitions into the test program before the body.  I think this is the
most appropriate thing to be doing, since the code itself uses LINUX and
not __linux__.