Check for unified addressing support in fallback_allocator example.
Check for unified addressing support in fallback_allocator example.

Fixes #406

xiongzhanblake/thrust_src,xiongzhanblake/thrust_src,zhenglaizhang/thrust,jaredhoberock/thrust,jaredhoberock/thrust,thrust/thrust,andrewcorrigan/thrust-multi-permutation-iterator,xiongzhanblake/thrust,zeryx/thrust,mohamed-ali/thrust,sarvex/thrust,GrimDerp/thrust,mohamed-ali/thrust,andrewcorrigan/thrust-multi-permutation-iterator,GrimDerp/thrust,thrust/thrust,andrewcorrigan/thrust-multi-permutation-iterator,marksantos/thrust,sdalton1/thrust,sdalton1/thrust,Ricardo666666/thrust,mohamed-ali/thrust,thvasilo/thrust,thvasilo/thrust,sarvex/thrust,egaburov/thrust,thrust/thrust,sdalton1/thrust,thvasilo/thrust,dachziegel/thrust,thrust/thrust,sarvex/thrust,arnabgho/thrust,zhenglaizhang/thrust,egaburov/thrust,zhenglaizhang/thrust,xiongzhanblake/thrust,marksantos/thrust,marksantos/thrust,raygit/thrust,dachziegel/thrust,raygit/thrust,raygit/thrust,arnabgho/thrust,egaburov/thrust,thrust/thrust,xiongzhanblake/thrust_src,GrimDerp/thrust,jaredhoberock/thrust,dachziegel/thrust,Ricardo666666/thrust,arnabgho/thrust,jaredhoberock/thrust,Ricardo666666/thrust,zeryx/thrust,zeryx/thrust,jaredhoberock/thrust,xiongzhanblake/thrust