Add AM_PROG_AR to configure.ac

Reported by intrigeri <intrigeri@boum.org>

Fixes #27

Signed-off-by: Ian Goldberg <iang@cs.uwaterloo.ca>

configure.ac

@@ -6,6 +6,8 @@ AM_CONFIG_HEADER(config.h)
 
 AM_INIT_AUTOMAKE([-Wall -Werror])
 
+m4_ifdef([AM_PROG_AR], [AM_PROG_AR])
+
 AC_PROG_CC
 
 dnl We do not want to create a .a for the plugin, so disable by default.