configure: only use AM_PROG_AR when available

This fixes commit 78d189fb on systems with older automake ...

diff --git a/configure.ng b/configure.ng
index 3ac56c4..8b47ad1 100644 (file)
--- a/configure.ng
+++ b/configure.ng
@@ -60,7 +60,8 @@ AC_PROG_CC_STDC
 
 # -- Helper programs --
 
-AM_PROG_AR
+m4_ifdef([AM_PROG_AR], [AM_PROG_AR])
+
 AC_PROG_AWK
 AC_PROG_INSTALL
 AC_PROG_LN_S