Make cusolver finding work for really old versions of cmake with CUDA 10.1

dlib/CMakeLists.txt

@@ -720,6 +720,16 @@ if (NOT TARGET dlib)
             if (NOT CUDA_cusolver_LIBRARY)
                get_filename_component(cuda_blas_path "${CUDA_CUBLAS_LIBRARIES}" DIRECTORY)
                find_library(CUDA_cusolver_LIBRARY cusolver HINTS ${cuda_blas_path})
+               # CUDA 10.1 doesn't install symbolic links to libcusolver.so in
+               # the usual place.  This is probably a bug in the cuda
+               # installer.  In any case, If we haven't found cusolver yet go
+               # look in the cuda install folder for it.  New versions of cmake
+               # do this correctly, but older versions need help.
+               if (NOT CUDA_cusolver_LIBRARY)
+                  find_library(CUDA_cusolver_LIBRARY cusolver HINTS 
+                     /usr/local/cuda/lib64/
+                     )
+               endif()
                mark_as_advanced(CUDA_cusolver_LIBRARY)
             endif()
             # Also find OpenMP since cuSOLVER needs it.  Importantly, we only