Auto merge of #1038 - Amanieu:fix_netbsd, r=alexcrichton

Fix netbsd build with "stdbuild"

A bug was introduced in #930 which caused netbsd to not link to libm.

r? @alexcrichton

src/unix/mod.rs

@@ -306,11 +306,11 @@ cfg_if! {
     } else if #[cfg(target_os = "emscripten")] {
         #[link(name = "c")]
         extern {}
-    } else if #[cfg(all(target_os = "netbsd"))] {
+    } else if #[cfg(all(target_os = "netbsd",
+                        feature = "stdbuild", 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.
-        #[cfg_attr(feature = "stdbuild", cfg(target_vendor = "rumprun"))]
         #[link(name = "m")]
         extern {}
     } else if #[cfg(any(target_os = "macos",