If you are getting these warning in the build output in your Xamarin solution, then you probably need to install the latest version of the Java SDK.
The key here is to install the 32-bit version, NOT the 64-bit one like I did. Probably a good idea to remove any existing versions first.
Major version 51 relates to JDK 7, but later versions should also resolve this problem.
You can find the last version of the JDK (at time of writing) here http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html