stdenv: fix self.cc check

Since ENV.cc/cxx are accessors to ENV, any values being passed through
are coerced into strings, which means that a nil `ENV.cc` is actually
an empty string - which isn't considered to be false in Ruby.

Fixes mistydemeo/tigerbrew#161.
Fixes mistydemeo/tigerbrew#135.

Library/Homebrew/extend/ENV/std.rb

@@ -131,7 +131,7 @@ module Stdenv
     self.cc  = MacOS.locate("gcc-4.2")
     self.cxx = MacOS.locate("g++-4.2")
 
-    unless cc
+    if cc.empty?
       self.cc  = "#{HOMEBREW_PREFIX}/bin/gcc-4.2"
       self.cxx = "#{HOMEBREW_PREFIX}/bin/g++-4.2"
       raise "GCC could not be found" unless File.exist? cc