autoconf 2.61 / m4 1.4.7 don't define the macro, AC_PROG_AR. This needs to be
defined in configure.ac, so those versions will continue to function properly.
diff --git a/configure.ac b/configure.ac
index 1a0d788..55d9f45 100644
--- a/configure.ac
+++ b/configure.ac
@@ -16,6 +16,9 @@
 AC_CANONICAL_HOST
 
 AC_PROG_CC
+# <= autoconf 2.61 doesn't have AC_PROG_AR, but 2.63 has it. Not sure about
+# 2.62.
+AC_DEFUN([AC_PROG_AR], [AC_CHECK_TOOL(AR, ar, :)])
 AC_PROG_AR
 AC_PROG_LEX
 AC_PROG_RANLIB