auto merge of #8188 : huonw/rust/cfg-macro, r=pcwalton

Example:

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

Closes #8130.

diff --cc src/libsyntax/ext/base.rs
index 90e1c7db91a5060c74403ec422d8ecbe83b6a84f,64130b0f83337fa43f9fb450a7e4c01cc2112237..1dda2493da20a86c57417fa3857fa34fefa24ff0
--- 1/src/libsyntax/ext/base.rs
--- 2/src/libsyntax/ext/base.rs
+++ b/src/libsyntax/ext/base.rs
@@@ -197,8 -197,12 +197,10 @@@ pub fn syntax_expander_table() -> Synta
      syntax_expanders.insert(intern(&"module_path"),
                              builtin_normal_tt(
                                  ext::source_util::expand_mod));
 -    syntax_expanders.insert(intern(&"proto"),
 -                            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));

diff --cc src/libsyntax/syntax.rs
Simple merge