`fails_with` just blacklists compilers we know don't work - that's true for Apple's GCC 4.0 and 4.2. We also support building with other GCCs, and once I make some improvements there the appropriate statements can be added. That way it will fall back to an FSF GCC 4.7+ that the user has installed if they don't have a compatible compiler.

Given the Lion issues perhaps it also needs

```ruby
fails_with :clang do
  build 425
  cause "clang 3.2 or newer is required to build capnproto"
```

Which would blacklist Xcode 4.6 and earlier while allowing Xcode 5.