syntax: implement cfg!() which evaluates to true/false where #[cfg] would keep/remove.

Example:

   if cfg!(test) {
      calculation_to_run_only_when_testing();
   }

src/libsyntax/ext/base.rs

@@ -201,6 +201,8 @@ pub fn syntax_expander_table() -> SyntaxEnv {
                             builtin_item_tt(ext::pipes::expand_proto));
     syntax_expanders.insert(intern(&"asm"),
                             builtin_normal_tt(ext::asm::expand_asm));
+    syntax_expanders.insert(intern(&"cfg"),
+                            builtin_normal_tt(ext::cfg::expand_cfg));
     syntax_expanders.insert(
         intern(&"trace_macros"),
         builtin_normal_tt(ext::trace_macros::expand_trace_macros));