Ignore unknown variables since the way we were handling it broke the tests=regex option
Ignore unknown variables since the way we were handling it broke the
tests=regex option

Allow additional architecture options for the arch command line variable
Set the default architecture to sm_20

Fixes #509

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