Fix for nvrtc issues on Ubuntu 20.04 and newer CUDA releases

[mandar2812]

This might also possibly address related issues like #11

With newer Ubuntu versions like 20.04 installing the cuda packages via apt sends them to /lib/x86_64-linux-gnu.

[korjaa]

I was able to fix my Ubuntu 20.04 compilation with the following diff:

diff --git a/libethash-cuda/CMakeLists.txt b/libethash-cuda/CMakeLists.txt
index 438fa8134..c26d84222 100644
--- a/libethash-cuda/CMakeLists.txt
+++ b/libethash-cuda/CMakeLists.txt
@@ -49,10 +49,7 @@ file(GLOB headers CUDAMiner.h CUDAMiner_cuda.h ${CMAKE_CURRENT_BINARY_DIR}/CUDAM
 
 cuda_add_library(ethash-cuda STATIC ${sources} ${headers})
 add_dependencies(ethash-cuda cuda_kernel)
-# Cmake doesn't handle nvrtc automatically
-find_library(CUDA_nvrtc_LIBRARY NAMES nvrtc PATHS ${CUDA_TOOLKIT_ROOT_DIR} PATH_SUFFIXES lib64 lib/x64 lib64/stubs lib/x64/stubs lib NO_DEFAULT_PATH)
-find_library(CUDA_cuda_LIBRARY NAMES cuda PATHS ${CUDA_TOOLKIT_ROOT_DIR} PATH_SUFFIXES lib64 lib/x64 lib64/stubs lib/x64/stubs lib NO_DEFAULT_PATH)
 target_link_libraries(ethash-cuda ethcore ethash::ethash progpow Boost::thread)
-target_link_libraries(ethash-cuda ${CUDA_nvrtc_LIBRARY} ${CUDA_cuda_LIBRARY})
+target_link_libraries(ethash-cuda nvrtc cuda)
 target_include_directories(ethash-cuda PUBLIC ${CUDA_INCLUDE_DIRS})
 target_include_directories(ethash-cuda PRIVATE .. ${CMAKE_CURRENT_BINARY_DIR})

But I guess it might break earlier builds that do need special handling.