Restore compatibility with older autoconfs.

The one in Ubuntu 10.04 doesn't know what AM_PROG_AR means, so
configure.ac was broken in r10053 when fixing compatibility with later
versions; you can't win...

[originally from svn r10086]
[r10053 == 2d9cc79d53]

unix/configure.ac

@@ -14,7 +14,7 @@ AM_INIT_AUTOMAKE([-Wall -Werror foreign])
 
 AC_PROG_INSTALL
 AC_PROG_RANLIB
-AM_PROG_AR
+ifdef([AM_PROG_AR],[AM_PROG_AR])
 AM_PROG_CC_C_O
 
 # Mild abuse of the '--enable' option format to allow manual