Fix build on musl

[FauxFaux]

Use a macro to avoid specifying a type for our integer constant, allowing the compiler to pick the right option for musl and non-musl.

The tests pass, but I haven't convinced myself it is actually doing the right thing.

strace seems happy:

[pid 26431] ioctl(9, BTRFS_IOC_CLONE or FICLONE, 5) = 0

Fixes #4.

[FauxFaux]

Simply adding - cargo test --target=x86_64-unknown-linux-musl --verbose --all --no-fail-fast -- --nocapture to the script: has comically broken travis, because of Windows in the matrix. I have removed it. I think it should be tested, but I'm not in the mood for rewriting the whole travis matrix right now.

The target isn't even installed, so that needs handling too.

[nicokoch]

Thanks!