Add :expat special dependency

This will be expanded to a full expat dependency on Tiger alone.

Fixes mistydemeo/tigerbrew#30.

Library/Homebrew/dependency_collector.rb

@@ -117,6 +117,8 @@ class DependencyCollector
     when :emacs      then EmacsRequirement.new(tags)
     # Tiger's ld is too old to properly link some software
     when :ld64       then LD64Dependency.new if MacOS.version < :leopard
+    # Tiger doesn't ship expat in /usr/lib
+    when :expat      then Dependency.new("expat", tag) if MacOS.version < :leopard
     when :python2
       PythonRequirement.new(tags)
     else