configure.ac: use 64-bit file API on 32-bit linux

Without the change graphviz fails to run when built against i686-linux target and installed on thew filesystem with 64-bit inodes (btrfs with many files in my case):

$ dot a.dot -T png > a.png
Format: "png" not recognized. No formats found.
Perhaps "dot -c" needs to be run (with installer's privileges) to register the plugins?

With the change it runs as expected:

$ dot a.dot -T png > a.png

This happens because 32-bit file APIs skip or error out files that require 64-bit values in inode numbers, sizes or similar places defined by off_t type.

The change enables the respective macros to enable 64-bit API.