Use cfg_attr to gate musl linking

Fixes rust-lang#684. Previously building libc for musl would fail, because we don't want to use #[link(...)] outside a stdbuild. This commit fixes that issue. (Although it may leave other targets broken, I'm not sure.)

Author: Others

src/unix/mod.rs

@@ -236,8 +236,8 @@ cfg_if! {
         // cargo build, don't pull in anything extra as the libstd  dep
         // already pulls in all libs.
     } else if #[cfg(any(all(target_env = "musl", not(target_arch = "mips"))))] {
-        #[link(name = "c", kind = "static", cfg(target_feature = "crt-static"))]
-        #[link(name = "c", cfg(not(target_feature = "crt-static")))]
+        #[cfg_attr(stdbuild, link(name = "c", kind = "static", cfg(target_feature = "crt-static")))]
+        #[cfg_attr(stdbuild, link(name = "c", cfg(not(target_feature = "crt-static"))))]
         extern {}
     } else if #[cfg(target_os = "emscripten")] {
         #[link(name = "c")]