Add config option to enable building support Unity (Both 32 & 64 bit)

This allows for a extra config option to enable building the Unity integration extension created by @FreeleX. The extension adds sound menu integration as well as notifications.

Previously, the extension only functioned on 32 bit machines. Compiling the extension with NG allows both 32 and 64 bit versions function properly. The major reason is the 32 and 64 bit versions have their own respective arch dependencies which are already present in the build system, while the add-on posted on the NG site was only compiled against a 32 bit SDK (I assume that's what happened).

From a clean build, the extension builds, installs by default, and functions correctly (Note: I haven't had a chance to test a build on a 32 bit version yet, but this does work for a build on my 64 bit machine).

It certainly could use some more testing, but it is functional (for x86_64, at least) currently.
When I get a chance, I'll build and test it on a 32 bit machine.

Add-on's [thread](http://forum.getnightingale.com/thread-554.html) in NG forum, and its [source](https://github.com/FreeleX/UnityIntegration)