SASL auth failure, reported by Cyrus SASL as "no mechanism available"

When I run 'make svnserveautocheck', I'm getting the following error
for pretty much all tests which use ra_svn:

  svn: SASL(-4): no mechanism available: No worthy mechs found

This problem occurs when libsvn_ra_svn/sasl_auth.c's try_auth()
detects SASL_NOMECH, either from sasl_client_start() or its
handle_interact() function.

Malcolm reported the same problem back in last October, and Vlad
responded <http://svn.haxx.se/dev/archive-2006-10/0317.shtml> saying
that this error can occur if Cyrus auth plug-ins are missing from your
/usr/lib/sasl2 directory. For Malcolm's case, Vlad suggested that a
missing libanonymous.so was the problem. I have such a plug-in. What
else could be causing this problem?

Here's the contents of my /usr/lib/sasl2 directory:

libanonymous.la* liblogin.so.2@ libsasldb.la*
libanonymous.so@ liblogin.so.2.0.21* libsasldb.so@
libanonymous.so.2@ libplain.la* libsasldb.so.2@
libanonymous.so.2.0.21* libplain.so@ libsasldb.so.2.0.21*
liblogin.la* libplain.so.2@ smtpd.conf
liblogin.so@ libplain.so.2.0.21*

Here's more of the output from a test failure (authz_tests.py):

START: authz_tests.py
CMD: svnadmin "create" "svn-test-work/local_tmp/repos" "--bdb-txn-nosync" <TIME = 0.103815>
CMD: svn "import" "--username" "jrandom" "--password" "rayjandom" "-m" "Log message for revision 1." "svn-test-work/local_tmp/greekfiles" "svn://127.0.0.1/svn-test-work/local_tmp/repos" "--config-dir" "/home/dlr/src/subversion/subversion/tests/cmdline/svn-test-work/local_tmp/config" <TIME = 0.160387>
subversion/libsvn_ra_svn/sasl_auth.c:345: (apr_err=170001)
svn: SASL(-4): no mechanism available: No worthy mechs found
Traceback (most recent call last):
  File "/home/dlr/src/subversion/subversion/tests/cmdline/authz_tests.py", line 812, in ?
...
END: authz_tests.py