Build failure on alpine

[jprendes]

wasmedge-rust-sdk fails to build on alpine since the libc crate doesn't have libc::pthread_sigqueue for musl targets.

error[E0425]: cannot find function `pthread_sigqueue` in crate `libc`
311.1     --> /usr/local/cargo/registry/src/index.crates.io-6f17d22bba15001f/wasmedge-sys-0.17.1/src/executor.rs:38:19
311.1      |
311.1 38   |             libc::pthread_sigqueue(dist_pthread, sig, si_value);
311.1      |                   ^^^^^^^^^^^^^^^^ help: a function with a similar name exists: `pthread_sigmask`
311.1      |
311.1     ::: /usr/local/cargo/registry/src/index.crates.io-6f17d22bba15001f/libc-0.2.147/src/unix/linux_like/linux/mod.rs:4623:5
311.1      |
311.1 4623 |     pub fn pthread_sigmask(how: ::c_int, set: *const sigset_t, oldset: *mut sigset_t) -> ::c_int;
311.1      |     -------------------------------------------------------------------------------------------- similarly named function `pthread_sigmask` defined here
311.1 
313.4 For more information about this error, try `rustc --explain E0425`.

Could this be gated behind a feature flag, or turned off for musl libc?