Fix static wasm build without wasi

[tbu-]

libc doesn't have off_t for WebAssembly without WASI, so it falls
back to long. Do the same in this crate to fix the build error when
trying to statically build libz for WebAssembly.

Rough code from zconf.h declaring z_off_t:

#ifndef Z_SOLO
#  if defined(Z_HAVE_UNISTD_H) || defined(_LARGEFILE64_SOURCE)
// [...]
#    ifndef z_off_t
#      define z_off_t off_t
#    endif
#  endif
#endif

// [...]

#ifndef z_off_t
#  define z_off_t long
#endif

Fixes #95.

[joshtriplett]

@tbu- I'd prefer not to special-case WebAssembly here. Could you, instead, submit a PR to the libc crate defining off_t for WebAssembly targets?

[tbu-]

@joshtriplett There's no off_t for WebAssembly, as far as I understand it. off_t is defined by POSIX, and WebAssembly doesn't implement POSIX. zlib defines off_t to be long when off_t isn't defined. It seems to me that the handling needs to be in zlib-sys.

[joshtriplett]

@tbu- AFAICT WASI defines filesize and filedelta, which provide the equivalents of off_t. So it's defined that offsets are always 64-bit.

[tbu-]

WASI does indeed define off_t:

https://github.com/rust-lang/libc/blob/f6df53fd694f6fc903058c765efc10d77725b31b/src/wasi.rs#L23

This is only included when targeting WASI though, not on all of WebAssembly:

https://github.com/rust-lang/libc/blob/f6df53fd694f6fc903058c765efc10d77725b31b/src/lib.rs#L150-L156

But WASI is just a possible environment of WebAssembly, libz also works without WASI, and in this case libz-sys (but not libz itself) fails to build. libz falls back to C's long type if off_t isn't present. Since libz does that, libz-sys probably should as well?

[joshtriplett]

@tbu- Then in that case, can you please modify this to only apply for wasm targets with OS "unknown", but not WASI targets?

[tbu-]

The target_env of unknown targets is actually the empty string "" and not "unknown". :o

[tbu-]

@tbu- Then in that case, can you please modify this to only apply for wasm targets with OS "unknown", but not WASI targets?

Done. Thanks for your patience and working with me. :) The CI failure is unrelated, it's a 500 error from crates.io.

Should I add a CI job testing this? It works locally.

[tbu-]

Done.

By experimenting, I found that I need target_os = "unknown" instead of target_os = "none".

[joshtriplett]

Done.

By experimenting, I found that I need target_os = "unknown" instead of target_os = "none".

Right, good catch. Mixed it up with x86_64-unknown-none. 🤦