chcon: link to libfts on musl targets

[ctrlcctrlv]

This fixes a build failure on aarch64-unknown-linux-musl.

On musl‐based targets the fts API is not provided by libc but by a separate libfts library.

src/uu/chcon/build.rs

Add a build script (build.rs) that detects when CARGO_CFG_TARGET_OS=linux && CARGO_CFG_TARGET_ENV=musl and emits rustc-link-lib=fts, so that uu_chcon can link successfully.

src/uu/chcon/src/fts.rs

In the glibc version of the header, level is short, under musl it's int; likewise, pathlen is unsigned on musl but signed on glibc.

Note

To test this, I build as: RUSTFLAGS='-Ctarget-feature=-crt-static' make in Alpine Linux.

[github-actions[bot]]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)

[sylvestre]

i am not sure creating a build.rs is necessary

[ctrlcctrlv]

How else would you conditionally link the -lfts library?