Support a command-line build option to choose the backend.
Support a command-line build option to choose the backend.

h1arshad/thrust,rdmenezes/thrust,lishi0927/thrust,bfurtaw/thrust,google-code-export/thrust,hemmingway/thrust,julianromera/thrust,bfurtaw/thrust,UIKit0/thrust,bfurtaw/thrust,allendaicool/thrust,allendaicool/thrust,rdmenezes/thrust,malenie/thrust,UIKit0/thrust,hemmingway/thrust,lishi0927/thrust,allendaicool/thrust,malenie/thrust,malenie/thrust,levendlee/thrust,google-code-export/thrust,Vishwa07/thrust,julianromera/thrust,Vishwa07/thrust,julianromera/thrust,rdmenezes/thrust,levendlee/thrust,lishi0927/thrust,hemmingway/thrust,lishi0927/thrust,malenie/thrust,h1arshad/thrust,allendaicool/thrust,h1arshad/thrust,levendlee/thrust,bfurtaw/thrust,hemmingway/thrust,Vishwa07/thrust,UIKit0/thrust,levendlee/thrust,Vishwa07/thrust,google-code-export/thrust,rdmenezes/thrust,UIKit0/thrust,google-code-export/thrust,julianromera/thrust,h1arshad/thrust