Support automake-1.15

Without this, autoreconf with automake-1.15 fails due to the "-Werror"
diff --git a/configure.ac b/configure.ac
index cdfe49a..0d8317b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -7,6 +7,7 @@
 
 AM_MAINTAINER_MODE
 
+AM_PROG_AR
 AM_PROG_LIBTOOL
 
 dnl Checks for programs.