add haiku support

[hoanga]

hello,

the following changes adds compilation support for haiku. note that this relies on a more recent mainline of the nix dependency in order for this to be able to compile.

a sample run is included below:

> uname -a
Haiku shredder 1 hrev56098 May 13 2022 08:04:13 x86_64 x86_64 Haiku

> cargo test
    Finished test [unoptimized + debuginfo] target(s) in 0.30s
     Running unittests (target/debug/deps/ctrlc-924088b04a373e44)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running src/tests.rs (target/debug/deps/tests-01b40067d85964c8)

test tests::test_set_handler ... ok

   Doc-tests ctrlc

running 2 tests
test src/lib.rs - set_handler (line 68) - compile ... ok
test src/lib.rs - (line 25) - compile ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.12s

[Detegr]

Thanks. First time I've heard of Haiku :) I'll wait for the next nix release before merging this, as I don't want the crate to rely on unreleased branch.

[Detegr]

Merged this manually now that Nix 0.25 is released and this crate is using it.