configure.ac: fix for AM_PROG_AR

AM_PROG_AR is not available with some versions of automake but is required by some others.

Fix provided by Brice Goglin (all rights reserved).

configure.ac

@@ -59,7 +59,7 @@ m4_ifdef([AM_SILENT_RULES],
 
 AC_PREREQ(2.60)
 
-AM_PROG_AR
+m4_ifdef([AM_PROG_AR], [AM_PROG_AR])
 AC_PROG_CC
 AM_PROG_CC_C_O
 AC_PROG_CXX