treating warnings as errors causing issues on Windows

Currently, for debug builds warnings are treated as errors as per:
https://github.com/andrewrk/libsoundio/blob/master/CMakeLists.txt#L201

However, there are also currently a lot of warnings on the Windows build. Could this be disabled for now until the actual warnings can be fixed? Once fixing them, it seems it'd be nice to enable that for all configurations to avoid surprises when swapping configurations.