In `musl` libc, defining `_GNU_SOURCE` doesn't automatically define the
other feature definitions, causing that `uv_fs_fstat` would not fill the
`nsec` fields. For the same reason, compile the tests on linux with
`-D_GNU_SOURCE` so the `fs_fstat` tests passes on `musl` platforms.
PR-URL: https://github.com/libuv/libuv/pull/1039
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
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