The python script provides a method to get the repository of mingw-builds
gcc compilers and download one of them. This is useful for providing a
matrix of compilations on appveyor.

The versions of compilers are seperated by multiple things:

  - version
  - threading model
  - exception model
  - revision

All four of those things need to be matched if using the libraries built
by MinGW. The script allows you to specify all of those variations. If
a variation isn't defined it picks the most common or latest settings.

For example, if the version isn't specified the latest will be selected
and if the exception model isn't defined then the zero exception model
(seh) will be selected if available.