Fix bug when building wine without X11

This was caused by `MacOS::X11.lib` raising an exception because of the `MacOS::X11#prefix` method: the `prefix` instance variable stays nil when neither `/opt/X11/lib/libpng.dylib` nor `/usr/X11/lib/libpng.dylib` exist.

Here's the output before this fix:

```
brew install wine --without-x11 -v
[snip]
Error: undefined method `/' for nil:NilClass
Please report this bug:
    https://github.com/mxcl/homebrew/wiki/troubleshooting
/usr/local/Library/Homebrew/os/mac/xquartz.rb:115:in `lib'
/usr/local/Library/Formula/wine.rb:89:in `wine_wrapper'
/usr/local/Library/Formula/wine.rb:143:in `install'
/usr/local/Library/Homebrew/build.rb:176:in `install'
/usr/local/Library/Homebrew/formula.rb:266:in `brew'
/usr/local/Library/Homebrew/formula.rb:617:in `stage'
/usr/local/Library/Homebrew/resource.rb:56:in `stage'
/usr/local/Library/Homebrew/extend/fileutils.rb:21:in `mktemp'
/usr/local/Library/Homebrew/resource.rb:53:in `stage'
/usr/local/Library/Homebrew/formula.rb:615:in `stage'
/usr/local/Library/Homebrew/formula.rb:261:in `brew'
/usr/local/Library/Homebrew/build.rb:155:in `install'
/usr/local/Library/Homebrew/build.rb:45:in `main'
/usr/local/Library/Homebrew/build.rb:12
/usr/local/Library/Formula/wine.rb:149
```