configure.ac: handle libraries installed in x86_64-linux-gnu

On some systems (Ubuntu 12.04) libpcap ends up installed in
/usr/lib/x86_64-linux-gnu. Update configure.ac to handle this.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Update configure scripts

Run ./autogen.sh to regenerate configure scripts after the change to
configure.ac.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Merge pull request #58 from cpackham/x86_64

X86 64