Some additions

I don't know if you're still planning on maintaining this, but if you are I've got some commits. I also pulled ccfreak's stuff, and merged it so it's all building fine. For the record, I'm building this with cc65 v2.13.9.20120809 on Windows.

Anyway, here's what I added:
    - Made a rudimentary version of write.c, and the small changes required for it. Printf and related functions are working.
    - Changed up the Makefiles to be compatible with GNU Make on Windows, and fixed dependency issues in a few places.
    - Added a bunch of tests for the different standard library headers. All the ones I've written so far work as expected, except some minor problems in ctypes.h. I've submitted a bug report to the cc65 people about that.
    - Merged ccfreak's dio stuff.
    - Probably some minor things I'm forgetting.

Github thinks I changed all the upstream files, but they should be untouched.

Cheers