Fix 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
Backport libevent to vanilla Autoconf 2.59 (as used in RHEL5)

This is a backport of ad03952.
Merge remote-tracking branch 'origin/pr/36' into patches-2.0