Fix compiletest on windows

Author: brson

src/compiletest/procsrv.rs

@@ -26,7 +26,7 @@ fn target_env(lib_path: ~str, prog: ~str) -> ~[(~str,~str)] {
 
     // Make sure we include the aux directory in the path
     assert!(prog.ends_with(~".exe"));
-    let aux_path = prog.slice(0u, prog.len() - 4u) + ~".libaux";
+    let aux_path = prog.slice(0u, prog.len() - 4u).to_owned() + ~".libaux";
 
     env = do vec::map(env) |pair| {
         let (k,v) = *pair;