(for master) workaround for missing AC_PROG_SED on older Autoconfs

For pre-2.59b Autoconfs, AC_PROG_SED is not available [1]; on such
systems, avoid calling AC_PROG_SED, while providing a sensible SED.

This aids backporting to Autoconf 2.59.

[1] http://lists.gnu.org/archive/html/autotools-announce/2004-08/msg00002.html