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.
Merge remote-tracking branch 'Homebrew/master'