std: Prepare for linking to musl

This commit modifies the standard library and its dependencies to link correctly
when built against MUSL. This primarily ensures that the right libraries are
linked against and when they're linked against they're linked against
statically.

src/libstd/sys/unix/time.rs

@@ -82,7 +82,8 @@ mod inner {
     // OpenBSD provide it via libc
     #[cfg(not(any(target_os = "android",
                   target_os = "bitrig",
-                  target_os = "openbsd")))]
+                  target_os = "openbsd",
+                  target_env = "musl")))]
     #[link(name = "rt")]
     extern {}