autoreconf is breaking in ArchLinux

Hi, Amir.

I've watched your interview on Plan B and got interested in your libbitcoin. But it's not building in ArchLinux. I have automake 1.13 here and since 1.12, it requests the use of the AM_PROG_AR macro to support more types of archivers.

So, I've made a very simple patch to make it possible to build on ArchLinux and here's the pull request. Hope it helps.