The cmake build of llvm in deps/Makefile might not be set up properly for msys yet. It may need to use a different cmake generator.
There is a bug in gcc 5 that causes it to miscompile llvm. This was fixed AIUI on gcc trunk which will hopefully become gcc 6 soonish. I'm not sure whether we have a workaround in place. Using a windows clang to build llvm in msys2 may also work but I think only Keno has tested that and he didn't write down many of the exact setup steps anywhere yet.
