Ensure nokogiri links against embedded zlib.
Use short versions of `mkmf` generated configure flags

`dir_config('foo')` sets up the following configure flags:

  --with-foo-dir
  --with-foo-include
  --with-foo-lib

More info at:

http://ruby-doc.org/stdlib-1.9.3/libdoc/mkmf/rdoc/Object.html#method-i-dir_config