fix: use CARGO_CFG_TARGET_OS and target_os android

[mxinden]

In a build.rs file cfg(target_os = "linux") refers to the target OS of the build.rs binary. The environment variable CARGO_CFG_TARGET_OS refers to the target OS of the final binary.

When cross-compiling, the target OS of the build.rs binary and the target OS of the final binary are not the same. The mtu's build.rs should use CARGO_CFG_TARGET_OS.

In addition, cfg(target_os = "linux") does not include android. This commit cfgs linux and android everywhere.

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Fixes #78.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.71%. Comparing base (1458645) to head (84ca63e).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #81   +/-   ##
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  Coverage   86.71%   86.71%           
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  Files           5        5           
  Lines         557      557           
  Branches      557      557           
=======================================
  Hits          483      483           
  Misses         48       48           
  Partials       26       26           

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[mxinden]

While I believe this is a step forward, this still needs more work. I will take another look tomorrow.

I wonder, does the usage of bindgen here even work for cross-compilation in the first place?

Recent build with this pull request: https://treeherder.mozilla.org/jobs?repo=try&revision=d606720cbe371d272648f8d80702d50416962d54&selectedTaskRun=K6LyBVWgR1iof9RZmC43rQ.0

[larseggert]

I think bindgen should work, if it finds the include files we tell it to ingest in the cross-environment for the target platform. I guess the question is "does it?" for moz-central...

[mxinden]

This is ready for review.