building PCL solutions with mdtool throws error

Hi,

I am trying to build a xamarin forms PCL solution using mdtool. Below is the command i used.

/Applications/XamarinStudio.app/Contents/MacOS/mdtool -v build "--configuration:Ad-Hoc|iPhone" /Users/s.chandraganesh/Downloads/Todo/PCL/Todo.iOS/Todo.iOS.csproj

I am getting the below error :

TextToSpeech_iOS.cs(10,34): error CS0246: The type or namespace name

ITextToSpeech' could not be found. Are you missing an assembly
reference?
    SQLite_iOS.cs(11,28): error CS0246: The type or namespace name

 1 Warning(s)
 2 Error(s)

The sample project I have downloaded from : https://developer.xamarin.com/samples/xamarin-forms/Todo/

Please help.