Initial MinGW support

This adds enough of @venix1 changes to make GDC and phobos build for MinGW and MinGW64 and allows to build 32/64bit hello world applications. No further testing has been done.

I do not really want to support a complete MinGW port, but once we get DDMD we'll need a basic MinGW GDC compiler to build the ```windows host --> other target``` cross compilers.

@venix1 Do you want to be added as the GIT_AUTHOR of these commits or shall I only add a comment
to the extended commit description? This is mostly your work after all.
Would be great if you could do a quick review as well, in case I screwed something up.

OT: @ibuclaw I published the configuration files for the binary builds at gdcproject.org/downloads here:
https://github.com/jpf91/gdc-build-configs/
If you want to you can fork this into the D-Programming-GDC organization.
There's still some stuff missing (most important: The build tool, website template and sqlite database of all builds, crosstool-NG patches). I'll publish that stuff later on.