Use sys_get_temp_dir(), not hardcoded "/tmp"

See: <phacility#55>

Author: epriestley

xhprof_lib/utils/xhprof_runs.php

@@ -95,8 +95,7 @@ public function __construct($dir = null) {
       $dir = ini_get("xhprof.output_dir");
       if (empty($dir)) {
 
-        // some default that at least works on unix...
-        $dir = "/tmp";
+        $dir = sys_get_temp_dir();
 
         xhprof_error("Warning: Must specify directory location for XHProf runs. ".
                      "Trying {$dir} as default. You can either pass the " .