[plt-scheme] MzScheme and Cygwin

The exp-tagged source for MzScheme (version 207.1) now builds with
Cygwin.

Unlike previous Cygwin builds of MzSchemes, the new makefiles produce
DLLs, just like the usual MSVC-based build.

However, the DLLs built by Cygwin and MSVC are not completely
interchangeable:

 - If you use a Cygwin-built mzscheme.exe, then it can use any
   combination of Cygwin-built and MSVC-built DLLS (libmzschxxxxxxx.dll
   and/or libmzgcxxxxxxx.dll).

 - If you start with a MSVC-built mzscheme.exe, then it works only if
   all the DLLs are MSVC-built.

Furthemore, if you start with a Cygwin-built mzscheme.exe, then Ron's
"arf.c" program works (even with the MSVC-built DLLs), but it doesn't
work if you start with a MSVC-built mzscheme.exe.

So, I'm getting closer. Meanwhile, if anyone knows how to make a Cygwin
DLL work in a non-Cygwin application, let me know!

Matthew