print full path;

--- a/src/Pure/System/build.scala	Tue Jul 31 16:26:12 2012 +0200
+++ b/src/Pure/System/build.scala	Tue Jul 31 17:40:33 2012 +0200
@@ -513,7 +513,7 @@
               (output_dir + log_gz(name)).file.delete
               File.write(output_dir + log(name), out)
               echo(name + " FAILED")
-              echo("(see also " + log(name).file.toString + ")")
+              echo("(see also " + (output_dir + log(name)).file.toString + ")")
               val lines = split_lines(out)
               val tail = lines.drop(lines.length - 20 max 0)
               echo("\n" + cat_lines(tail))