Fix cmake < v3.7 compatibility in Cuda.cmake (#6338)

Fix for compatibility with CMake <3.7 (related conversation under 37e4289)

* Fix cmake < v3.7 compatibility in Cuda.cmake

* Fix version test variable naming in Cuda.cmake

Author: vrichter

cmake/Cuda.cmake

@@ -113,8 +113,8 @@ function(caffe_select_nvcc_arch_flags out_variable)
   set(__nvcc_flags "")
   set(__nvcc_archs_readable "")
 
-  string(COMPARE GREATER_EQUAL "${CUDA_VERSION}" "9.0" iscudanewerthan90)
-  if(iscudanewerthan90)
+  string(COMPARE LESS "${CUDA_VERSION}" "9.0" iscudaolderthan90)
+  if(NOT iscudaolderthan90)
     string(REPLACE "21(20)" "" __cuda_arch_bin "${__cuda_arch_bin}")
     string(REPLACE "20" "" __cuda_arch_bin "${__cuda_arch_bin}")
   endif()