Perhaps building syscall on non-Redox platforms should error
Otherwise it is potentially possible to build a program using the library on other platforms, but it will not work correctly, and potentially cause undefined behaviour. The best way to do this is probably with a build.rs test.
I am somewhat concerned, though, that some software ported to Redox is probably depending unconditionally on the syscall crate, and this change would make it fail to build on other platforms. This is probably the right thing to do though, and those crates can be fixed (I guess we can test every crate on crates.io that depends on redox_syscall?).