When cross compiling with mingw it's problematic to assume that there is
a binary called windres, and having to set it via an environment
variable seems wrong when there is a handy cross-file for just such a
situation.
This patch allows setting windres in the [binaries] section of the cross
file. If the build is a cross build, then the windows module will check
for windres being set in the cross file before checking the WINDRES
environment variable or looking for a windres binary.
Everywhere we use this object, we end up iterating over it and comparing
compiler.get_language() with something. Using a dict is the obvious
choice and simplifies a lot of code.