curl-rust depends on mio-extras, which depends on mio-0.6 (and cannot use more recent versions of mio). But mio-0.6, which is quite old, cannot build with libc 0.2.176 or newer. In order to run curl-rust's test suite on FreeBSD, I suggest you either downgrade libc to 0.2.175 or else replace mio-extras with something more recent, perhaps mio-misc.
curl-rust depends on mio-extras, which depends on mio-0.6 (and cannot use more recent versions of mio). But mio-0.6, which is quite old, cannot build with libc 0.2.176 or newer. In order to run curl-rust's test suite on FreeBSD, I suggest you either downgrade libc to 0.2.175 or else replace mio-extras with something more recent, perhaps mio-misc.