ERROR: dynamic load not supported

All,
This is with a new 7.3.4 installation. Every CREATE FUNCTION call I
make, e.g.,

CREATE FUNCTION plpgsql_call_handler () RETURNS LANGUAGE_HANDLER AS
'<absolute-path-to-pg-install>/lib/plpgsql' LANGUAGE C;

fails with this error. I know the specification of absolute paths for
the library lookup is deprecated, but it's supposed to still work, and,
as of last week, did. "I haven't changed anything." :) (But doubtless
have done something stupid.)

Thanks for any advice.
Rich Cullingford
rculling(at)sysd(dot)com