[build] Add AM_PROG_AR

Newer automake / libtools warns otherwise.
diff --git a/configure.ac b/configure.ac
index b4249cd..061880f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -13,6 +13,7 @@
 AM_SILENT_RULES([yes])
 
 # Initialize libtool
+AM_PROG_AR
 LT_PREREQ([2.2])
 LT_INIT([disable-static])