avoid the error 'possibly undefined macro'
on autoconf --warnings=all,no-cross
diff --git a/configure.in b/configure.in
index 9de7217..160954c 100644
--- a/configure.in
+++ b/configure.in
@@ -8,7 +8,7 @@
 fi
 
 dnl Checks for programs.
-AM_PROG_AR
+m4_ifdef([AM_PROG_AR], [AM_PROG_AR])
 AC_PROG_RANLIB
 AC_PROG_CC
 AC_PROG_INSTALL