OpenBSD supports multiple versions of autoconf installed at the same time and uses a wrapper script (from the metaauto package) to invoke the correct one based on the environment variable AUTOCONF_VERSION. The rsync:HEAD build appears to be failing on configure because this variable isn't set.
The versions of autoconf available on OpenBSD 4.7 are: 2.13, 2.52, 2.54, 2.56, 2.57, 2.58, 2.59, 2.60, 2.61, and 2.62.
This applies to all of the other auto* tools such as automake, autoheader, etc.
We no longer use the build farm, nor autoconf etc.
Thanks for the bug report!