Blacklist creation for some formula

We don't duplicate, so libxml and that are already provided by OS X. Mercurial is provided by easy_install.

I'm adding this to save people contributing stuff that we won;t accept. I hate disappointing people :(

Library/Homebrew/brew.h.rb

@@ -85,7 +85,6 @@ def __make url, name
   return path
 end
 
-
 def make url
   path = Pathname.new url
 
@@ -103,6 +102,18 @@ def make url
     end
   end
 
+  case name
+  when /libxml/, /libxlst/, /freetype/, /libpng/
+    raise <<-EOS
+#{name} is blacklisted for creation
+Apple distributes this library with OS X, you can find it in /usr/X11/lib.
+However not all build scripts look here, so you may need to call ENV.x11 or
+ENV.libxml2 in your formula's install function.
+    EOS
+  when 'mercurial'
+    raise "Mercurial is blacklisted for creation because it is provided by easy_install"
+  end
+
   __make url, name
 end