Use target from env in build.rs

[ethanfrey]

Description

Cross compile is broken for wasmer-runtime-core. I tried compiling from linux -> macos (with lots of setup of dependencies first) and it compiled, but errored on the build.rs, which was using linux assembly instead of macos assembly.

Long explanation here to capture (and share) my learnings on this:

Digging into this, it turns out the reason was the use of if cfg!(all(target_os = "linux", target_arch = "x86_64")) { to check for the target in build.rs. This consistently detected the os as linux, even when I did cargo build --target x86_64-apple-darwin. However, the TARGET env variable was showing macos.

I thought this was a cargo bug, but as @nlewycky explained, build.rs is compiled for the host system (the one doing compilation), and correctly sets the target_os to linux. While the rest of the source code will have cfg(target_os) set to macos. The solution here is to rely on the CARGO_CFG_TARGET_{OS,ARCH} environmental variables in build.rs if you want to detect the system for which the library is being compiled for. (Normally they are the same, this bug only shows up during cross-compilation).

Review

I didn't add unit tests, here but am testing it when imported in another project where I have a cross-compile build system set up. I can link the example code here if you want to run.

• Add a short description of the the change to the CHANGELOG.md file

[ethanfrey]

To demo this working, you can check out the code from CosmWasm/wasmvm#14

and run make docker-image to build the cross-compile environment
and make docker-build to build for linux and osx.

Output should be api/libgo_cosmwasm.{so,dylib}. This compiling shows that the build.rs file now builds a valid output. If you have a recent version of go installed, you can try make test to ensure that the cross-compiled dylib actually behaves correctly. (A friend has manually verified this).

[nlewycky]

bors r+

[bors]

Merge conflict

[ethanfrey]

This was fixed by a different PR and included in the 0.11.0 release. Thanks for the update and now cross-compiling works much better.