Enable large file support

As of 1.16, xcb_util.c has several calls to stat on linux. On 32-bit systems these will fail with EOVERFLOW if called on files larger than (2^31)-1 bytes in size. Adding this line to configure.ac will redirect stat to stat64 which doesn't have this issue, along with adjusting a few other functions with similar issues. I think these callsites are only intended to handle unix sockets, so it shouldn't fail in practice, but it's better to avoid the issue entirely by calling the right version of stat in the first place.