aws-lc-sys build.rs ignores / overrides TARGET_CC / TARGET_CXX

[jschwe]

Problem:

cc-rs will respect TARGET_CC / TARGET_CXX when cross-compiling. However, aws-lc-sys sadly will override these user provided variables here, since the build script only configures the target specific variants (which cc-rs will prefer over the less specific TARGET_CC variable).

This can cause the build to fail when cross-compiling, the user set TARGET_CC to the correct cross-compiler, but aws-lc-sys overrode those.

Encountered in servo when cross-compiling for android: https://github.com/servo/servo/actions/runs/21467527086/job/61832707394#step:14:1106

[jschwe]

Presumably this can simply be fixed by checking for TARGET_<var> in optional_env_optional_crate_target()

[justsmth]

Thanks for digging into this!

warning: aws-lc-sys@0.37.0: Environment Variable found 'CC': 'clang'
warning: aws-lc-sys@0.37.0: Setting CC_x86_64_linux_android: clang
warning: aws-lc-sys@0.37.0: Environment Variable found 'CXX': 'clang++'
warning: aws-lc-sys@0.37.0: Setting CXX_x86_64_linux_android: clang++

I put up a PR that should prioritize TARGET_CC and TARGET_CXX over CC and CXX. It also adds some tests to help verify this logic.