mac: Don't define an SDKROOT.
mac: Don't define an SDKROOT.

This way Xcode will use the latest SDK, whatever it is, and we don’t
have to bump this every time Apple releases an OS update.

bbondy/brightray,atom/brightray,bbondy/brightray,brave/brightray,atom/brightray,brave/brightray,tejaspathak/brightray,tejaspathak/brightray