Using a more robust check to detect whether AutoNet is being built

if(TARGET AutoNet) is generally a more straightforward way to tell, within Autowiring, whether AutoNet is being built.  Checking flags like AUTOWIRING_BUILD_AUTONET or trying to check AutoNet's dependencies are both unreliable and fragile ways of accomplishing this.
Merge pull request #100 from leapmotion/bug-autonetlinkage

Using a more robust check to detect whether AutoNet is being built