Some systems manage to install cuda in a way that causes cmake to claim

that cuda is available even though it knows it didn't find cublas, which
is part of the standard cuda install.  So we need to add a check to see
if cmake *really for realz* found cuda.

dlib/CMakeLists.txt

@@ -615,6 +615,12 @@ if (NOT TARGET dlib)
             set(CUDA_FOUND 0)
          endif()
 
+         if (NOT CUDA_CUBLAS_LIBRARIES)
+            message(STATUS "Found CUDA, but CMake was unable to find the cuBLAS libraries that should be part of every basic CUDA "
+               "install. Your CUDA install is somehow broken or incomplete. Since cuBLAS is required for dlib to use CUDA we won't use CUDA.")
+            set(CUDA_FOUND 0)
+         endif()
+
          if (CUDA_FOUND AND (NOT USING_OLD_VISUAL_STUDIO_COMPILER))
 
             # There is some bug in cmake that causes it to mess up the