#238 generational GC on cygwin

[Sam Steingold]

cygwin build lacks generational GC
because the MMAP autoconf test fails:
<http://cygwin.com/ml/cygwin/2004-09/msg00741.html>

Since cygwin is layered on top of win32,
and CLISP already provides generational GC on win32,
there is no fundamental reason not to support it on cygwin
(not necessarily via cygwin mmap(),
maybe directly via win32).

[Reini Urban]

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OK, I'll start working on that.
--Reini