wrong .def file name building with mingw

[Torbjorn Granlund]

I tried to read your patch now, but I am afraid I cannot follow it.

I need explanatiion if each change, e.g., change logs.  Specific
questions:

What is this "resource file" thing?
What is the RC/windres stuff?

I believe people have reported that GMP worked under Mingw for both
x86-32 and x86-64.  Now, major changes seem to be needed.  Could you
explain this?  Has Mingw changed?

-- 
Torbjörn