Introduce a testcase for conditional compilation via attributes

Issue #489

src/test/run-pass/conditional-compile.rs

@@ -0,0 +1,10 @@
+// xfail-stage0
+// xfail-stage1
+// xfail-stage2
+
+// Since the bogus configuration isn't defined main will just be
+// parsed, but nothing further will be done with it
+#[cfg(bogus)]
+fn main() { fail }
+
+fn main() {}