Add back Caffe2_GPU to Caffe2_LINK variable if it can be enabled
Add back Caffe2_GPU to Caffe2_LINK variable if it can be enabled

davinwang/caffe2,davinwang/caffe2,pietern/caffe2,Yangqing/caffe2,bwasti/caffe2,sf-wind/caffe2,sf-wind/caffe2,pietern/caffe2,Yangqing/caffe2,davinwang/caffe2,Yangqing/caffe2,caffe2/caffe2,pietern/caffe2,xzturn/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,bwasti/caffe2,pietern/caffe2,bwasti/caffe2,davinwang/caffe2,bwasti/caffe2,sf-wind/caffe2,sf-wind/caffe2,sf-wind/caffe2,xzturn/caffe2,davinwang/caffe2,xzturn/caffe2,bwasti/caffe2,Yangqing/caffe2,pietern/caffe2