Carefully define signbit & isfinite
Carefully define signbit & isfinite

Fixes #502

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