Fix freebsd.

Author: Clar Charr

src/unix/mod.rs

@@ -239,7 +239,8 @@ cfg_if! {
     } else if #[cfg(target_os = "emscripten")] {
         #[link(name = "c")]
         extern {}
-    } else if #[cfg(all(target_os = "netbsd", target_vendor = "rumprun"))] {
+    } else if #[cfg(all(stdbuild,
+                        target_os = "netbsd", target_vendor = "rumprun"))] {
         // Since we don't use -nodefaultlibs on Rumprun, libc is always pulled
         // in automatically by the linker. We avoid passing it explicitly, as it
         // causes some versions of binutils to crash with an assertion failure.