Updated to work with Xamarin 6.4 / Mono 3.x.

Now that I understand the twisty maze of system DLL dependencies a bit more, I was able to
eliminate the five phase build process. However, getting NAnt to work with Mono 3.x (specifically
the .NET 4.5 profile) is more complicated. Hopefully NAnt will ship an update some time soon that
supports Mono 3.x. Not likely since their last update was over a year ago...
This hack is needed to get things working with 3.x.

It looks like the latest IKVM source just does the new guard that I introduced,
so maybe this is less of a hack than it seems.
Another hack I've had lying around which seems needed.