Fix compilation error.

--- configure.ac.orig	2013-03-25 04:52:48.000000000 +0100
+++ configure.ac	2013-03-25 17:03:53.000000000 +0100
@@ -22,6 +22,8 @@
 AM_INIT_AUTOMAKE([-Wall -Werror foreign nostdinc silent-rules])
 AM_MAINTAINER_MODE()
 
+AM_PROG_AR()
+
 LT_PREREQ([2.4])
 LT_INIT()