simplify cmake due to minimum project requirement already gone to 3.12

CMakeLists.txt

@@ -48,21 +48,18 @@ set(INSTALL_LIB_DIR      "${CMAKE_CURRENT_LIST_DIR}" CACHE PATH "Path where lib
 set(INSTALL_INCLUDE_DIR  "include/darknet"           CACHE PATH "Path where headers will be installed")
 set(INSTALL_CMAKE_DIR    "share/darknet"             CACHE PATH "Path where cmake configs will be installed")
 
-if(${CMAKE_VERSION} VERSION_LESS "3.9.0")
-  message(WARNING "To build with CUDA support you need CMake 3.9.0+")
-  set(ENABLE_CUDA "FALSE" CACHE BOOL "Enable CUDA support" FORCE)
-else()
 include(CheckLanguage)
 check_language(CUDA)
 if(CMAKE_CUDA_COMPILER AND ENABLE_CUDA)
   set(CUDA_ARCHITECTURES "Auto" CACHE STRING "\"Auto\" detects local machine GPU compute arch at runtime, \"Common\" and \"All\" cover common and entire subsets of architectures, \"Names\" is a list of architectures to enable by name, \"Numbers\" is a list of compute capabilities (version number) to enable")
   set_property(CACHE CUDA_ARCHITECTURES PROPERTY STRINGS "Auto" "Common" "All" "Kepler Maxwell Kepler+Tegra Maxwell+Tegra Pascal" "3.0 7.5")
   enable_language(CUDA)
-    find_package(CUDA REQUIRED)
-    if(CUDA_VERSION VERSION_LESS "9.0")
+  find_package(CUDAToolkit REQUIRED)
+  if(CMAKE_CUDA_COMPILER_VERSION VERSION_LESS "9.0")
     message(STATUS "Unsupported CUDA version, please upgrade to CUDA 9+. Disabling CUDA support")
     set(ENABLE_CUDA "FALSE" CACHE BOOL "Enable CUDA support" FORCE)
   else()
+    find_package(CUDA REQUIRED)
     cuda_select_nvcc_arch_flags(CUDA_ARCH_FLAGS ${CUDA_ARCHITECTURES})
     message(STATUS "Building with CUDA flags: " "${CUDA_ARCH_FLAGS}")
     if (NOT "arch=compute_70,code=sm_70" IN_LIST CUDA_ARCH_FLAGS)
@@ -73,7 +70,6 @@ else()
 else()
   set(ENABLE_CUDA "FALSE" CACHE BOOL "Enable CUDA support" FORCE)
 endif()
-endif()
 
 if (WIN32 AND ENABLE_CUDA AND CMAKE_MAKE_PROGRAM MATCHES "ninja")
   option(SELECT_OPENCV_MODULES "Use only few selected OpenCV modules to circumvent 8192 char limit when using Ninja on Windows" ON)