Musl and other libc implementation

[ghaith]

It would be nice to support other libc implementations in our standard lib.
It would also be nice to have a nostd version (Maybe by excluding the string functions)
This is a placeholder to discuss possible strategies regarding the standard lib

See #924

[corbanvilla]

Agree it would be beneficial to leave to the user to decide libc target, I've also run into cases where I need the glibc version.

[corbanvilla]

It appears that the issue with static linking glibc is the Rust standard lib, mentioned here.

These are the warnings I come up against when static linking with glibc, though it appears that none of these functions are actually used.

/usr/bin/ld: target/debug/libiec61131std.a(std-8389830094602f5a.std.fd8b77e9-cgu.0.rcgu.o): in function `std::sys::unix::os::home_dir::fallback':
/rustc/90c541806f23a127002de5b4038be731ba1458ca/library/std/src/sys/unix/os.rs:640: warning: Using 'getpwuid_r' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
/usr/bin/ld: target/debug/libiec61131std.a(std-8389830094602f5a.std.fd8b77e9-cgu.0.rcgu.o): in function `<std::sys_common::net::LookupHost as core::convert::TryFrom<(&str,u16)>>::try_from::{{closure}}':
/rustc/90c541806f23a127002de5b4038be731ba1458ca/library/std/src/sys_common/net.rs:206: warning: Using 'getaddrinfo' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking