gstep-base changes for MinGW/Windows

[Adam Fedor]

With the recent changes I've made to the base library, I've been able to
compile the base library (static library only) on MinGW for Windows, and
perhaps about 80% of the classes/test programs work.  

I can't seem to get the gstep-base DLL to work, though. It links fine,
but then gives a 'Can't find __objc_class_NSObject' error when running a
program.

So far, I've only done the compilation using Cygwin utilities to do the
compiling. I've added a README.MinGW in the core directory describing
how I did it.

-- 
Adam Fedor, Digital Optics            | Fudd's law of opposition: Push
fedor@doc.com  http://www.doc.com     | something hard enough, and it 
fedor@gnu.org  http://www.gnustep.org | will fall over.