Asserts Build Config Should Enable Upstream Asserts

[insertinterestingnamehere]

Just documenting this for now since I'm not sure yet where to make the changes.

There's a build configuration for turning on asserts. It should turn on upstream asserts (e.g. from libstdc++ or libc++) by setting appropriate CFLAGS and/or CXXFLAGS.

I also need to confirm the NDEBUG flag should also be set correctly for non-debug builds too. This avoids pulling in unwanted upstream asserts. That appears to be the case, but I haven't confirmed yet.