[plt-scheme] ifndef in PLT?

SRFI 7 *might* have what you want.
http://srfi.schemers.org/srfi-7/
-- 
... belief in the omniscient hacker is indistinguishable from
belief in a Supreme Being.  There is simply no argument one can
give that will dissuade a true believer, yet when the believer is
asked for a demonstration he is unable to produce one.
        -- Michael Shamos