Cross compiling for Windows and Darwin is not working.

[mattrust]

Building for Windows

runtime/cgo

gcc: error: unrecognized command-line option ‘-mthreads’; did you mean ‘-pthread’?

error building application: exit status 1
Building for Darwin

runtime/cgo

clang: error: unsupported option '-arch' for target 'x86_64-pc-linux-gnu'
clang: error: argument unused during compilation: '-mmacosx-version-min=10.11' [-Werror,-Wunused-command-line-argument]

error building application: exit status 1