[Bf-committers] enable 3D mouse for official builds

[Mike Erwin]

Hi devs,
During the switch to CMake, 3D mouse support was disabled by default on Mac
& Linux. Just want to make sure this is enabled for the 2.77 release.

Windows -- no dependencies
Mac -- no dependencies
Linux -- requires libspnav to build

I'm not very familiar with the install-deps script, which is why I'm
bringing this up instead of just fixing it!

Mike Erwin
musician, naturalist, pixel pusher, hacker extraordinaire