rpath the path to the runtime

We don't currently rpath native libs, but we do know where rustrt is
located and everything needs to link to it.

src/comp/back/rpath.rs

@@ -20,10 +20,14 @@ fn get_rpath_flags(sess: session::session, out_filename: str) -> [str] {
     let sysroot = sess.filesearch().sysroot();
     let output = out_filename;
     let libs = cstore::get_used_crate_files(sess.get_cstore());
+    // We don't currently rpath native libraries, but we know
+    // where rustrt is and we know every rust program needs it
+    let libs = libs + [get_sysroot_absolute_rt_lib(sess)];
+
     let target_triple = sess.get_opts().target_triple;
     let rpaths = get_rpaths(cwd, sysroot, output, libs, target_triple);
     rpaths_to_flags(rpaths);
-    [] // FIXME: activate RPATH!
+    [] // FIXME: Activate RPATH!
 }
 
 #[cfg(target_os="win32")]
@@ -31,6 +35,15 @@ fn get_rpath_flags(_sess: session::session, _out_filename: str) -> [str] {
     []
 }
 
+fn get_sysroot_absolute_rt_lib(sess: session::session) -> fs::path {
+    let path = [sess.filesearch().sysroot()]
+        + filesearch::relative_target_lib_path(
+            sess.get_opts().target_triple)
+        + [os::dylib_filename("rustrt")];
+    check vec::is_not_empty(path);
+    fs::connect_many(path)
+}
+
 fn rpaths_to_flags(rpaths: [str]) -> [str] {
     vec::map({ |rpath| #fmt("-Wl,-rpath,%s",rpath)}, rpaths)
 }