Set --host flag for autotools to support cross compilation

[skywhale]

https://docs.rs/autotools/0.2.1/autotools/struct.Config.html#method.host
It's set automatically, but based on the compilation host. We want the target architecture to be set for --host flag.

[bschwind]

LGTM!

[goodhoko]

@skywhale @bschwind Sorry to dig this up after 3 years. Let's finish this. .) Seems like we could just merge? If not, let's close this.

[bschwind]

We should probably test before merging though, by successfully cross-compiling this library with this branch.

[goodhoko]

I'll give it a try. I just need to make the regular compilation work first. (something something c++ errors out)

[goodhoko]

@bschwind Tested this by successfully compiling for x86 macOS on an arm macOS. You need to have have the target's rust toolchaing installed and you also need to set PKG_CONFIG_SYSROOT_DIR to this weird value (found out here)

I pushed an additional commit that documents this in the README.

PKG_CONFIG_SYSROOT_DIR=/ cargo build --target x86_64-apple-darwin 

[bschwind]

I think this is an incorrect assumption, as some targets can have 3 components, some can have 4, etc.

When testing it's important to pass the --features "bundled" flag so the C++ code is built from source.

brian webrtc-audio-processing-sys $ PKG_CONFIG_SYSROOT_DIR=/ cargo build --features "bundled" --target aarch64-apple-ios
   Compiling webrtc-audio-processing-sys v0.3.0 (/Users/brian/projects/tonari/webrtc-audio-processing/webrtc-audio-processing-sys)
error: failed to run custom build command for `webrtc-audio-processing-sys v0.3.0 (/Users/brian/projects/tonari/webrtc-audio-processing/webrtc-audio-processing-sys)`

Caused by:
  process didn't exit successfully: `/Users/brian/projects/tonari/webrtc-audio-processing/webrtc-audio-processing-sys/target/debug/build/webrtc-audio-processing-sys-e044a990654280ee/build-script-build` (exit status: 101)
  --- stderr
  thread 'main' panicked at build.rs:72:9:
  assertion `left == right` failed
    left: 3
   right: 2
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[goodhoko]

Good point!

When I use the bundled feature and a 4-component target I get a configure errors like:

  --- stderr
  configure: error: in `/Users/jentak/workspace/tonari/webrtc-audio-processing/target/aarch64-unknown-linux-musl/debug/build/webrtc-audio-processing-sys-7f94f1d16145798d/out/build':
  configure: error: C compiler cannot create executables

When I comment out https://github.com/tonarino/webrtc-audio-processing/pull/19/files#diff-4038f237c46940dc12666050b832d77271ddb0171c9bc64700009de2a982a403R106 (i.e. essentially ignore changes from this PR) and cross-compile for eg. x86_64-apple-darwin the build finishes successfully.

PKG_CONFIG_SYSROOT_DIR=/ cargo -v build --target x86_64-apple-darwin --features "bundled"

So I'm not sure if this PR is even needed? See also my comment below.

[goodhoko]

@skywhale

Do you remember why you wrote this?

We want the target architecture to be set for --host flag.

Yes, rust's "target" is "host" in autotools's nomenclature, but the autotools crate docs say:

The host and target methods on this package’s autotools::Config structure (as well as the $HOST and $TARGET variables set by cargo) are understood with their Rust meaning.

Anyway, I think I've lost too much time on this trying to debug opaque autoconf//gcc/cc errors. If anyone else won't pick this up let's close it as stale.