This doesn't work in a normal shell, but enables 'make -C mingw'
which is convenient.

Thanks to ron for the suggestion.

This builds win32 versions of the library and examples
on linux using the mingw-gcc cross toolchain and wine.

It also automates downloading and building the
required dependencies. Update the _URL and _SHA
variables to build against newer upstream releases.

Thanks to Ron and Mark Harris for help with the makefile.