Add DISABLE_UNSAFE_OPTS setting.

This adds a DISABLE_UNSAFE_OPTS setting.  It allows the compilation to explicitly opt-out of LLVM optimisations that are suspected to be unsafe, per the can_use_unsafe_opts() method that already exists in the code.

I used this to work around the compilation issue described here:

    https://groups.google.com/forum/#!topic/emscripten-discuss/HPMtJNVguWQ

It might be useful to expose this to help debugging/working-around similar issues in the future.