Fix Cache#mkdir_p to handle absolute paths - fix #5

lib/rscons/cache.rb

@@ -181,6 +181,7 @@ module Rscons
     def mkdir_p(path)
       parts = path.split(/[\\\/]/)
       parts.each_index do |i|
+        next if parts[i] == ""
         subpath = File.join(*parts[0, i + 1])
         unless File.exists?(subpath)
           FileUtils.mkdir(subpath)