Xp/benchmark app ocl (#12112)

* Add some tip description about enable OpenCL for benchmark_app.

Signed-off-by: xipingya <xiping.yan@intel.com>

* Export doesn't work, we need to add -Dopencl_root_hints=[PATH]/OpenCL-CLHPP/include to cmake command.

Signed-off-by: xipingya <xiping.yan@intel.com>

Co-authored-by: Chen Peter <peter.chen@intel.com>

samples/cpp/benchmark_app/CMakeLists.txt

@@ -22,7 +22,10 @@ else()
 endif()
 
 if(SAMPLES_ENABLE_OPENCL)
-    find_package(OpenCL)
+    find_package(OpenCL QUIET)
+    if(NOT OpenCL_FOUND)
+        MESSAGE(WARNING "OpenCL is disabled or not found, ${TARGET_NAME} will be built without OpenCL support. Install OpenCL.")
+    endif()
 
     find_path(OpenCL_HPP_INCLUDE_DIR
         NAMES
@@ -49,6 +52,8 @@ if(SAMPLES_ENABLE_OPENCL)
         # Append OpenCL CPP headers to C headers and use both
         set(OpenCL_HEADERS ${OpenCL_INCLUDE_DIR} ${OpenCL_HPP_INCLUDE_DIR})
         set(OpenCL_LIB "OpenCL::OpenCL")
+    else()
+        MESSAGE(WARNING "OpenCL CPP header is not found, ${TARGET_NAME} will be built without OpenCL support. Download it from: https://github.com/KhronosGroup/OpenCL-CLHPP and set -Dopencl_root_hints=[PATH]/OpenCL-CLHPP/include to cmake.")
     endif()
 
     if(OpenCL_FOUND AND OpenCL_HEADERS)