Fix Rustbuild linking on Illumos

Illumos (an OpenSolaris fork) expects to get several
extra library references for some system functions used
by Rust standard library. This commit adds required linker
options to rustbuild, which is currently doesn't work on
Illumos-based operating systems.

src/libstd/build.rs

@@ -41,6 +41,10 @@ fn main() {
     } else if target.contains("dragonfly") || target.contains("bitrig") ||
               target.contains("netbsd") || target.contains("openbsd") {
         println!("cargo:rustc-link-lib=pthread");
+    } else if target.contains("solaris") {
+        println!("cargo:rustc-link-lib=socket");
+        println!("cargo:rustc-link-lib=posix4");
+        println!("cargo:rustc-link-lib=pthread");
     } else if target.contains("apple-darwin") {
         println!("cargo:rustc-link-lib=System");