Allow --force for blacklisted formula names

Library/Homebrew/brew.h.rb

@@ -102,17 +102,21 @@ def make url
     end
   end
 
-  case name
+  force_text = "If you really want to make this formula use --force."
+
+  case name.downcase
   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.
+
+#{force_text}
     EOS
   when 'mercurial', 'scons'
-    raise "#{name} is blacklisted for creation because it is provided by easy_install"
+    raise "#{name} is blacklisted for creation because it is provided by easy_install.\n\n#{force_text}"
-  end
+  end unless ARGV.force?
 
   __make url, name
 end