From: Paul Zander <negril.nx+gentoo@gmail.com>

Find the system opencl and inject the include dir.

diff --git a/cmake/OpenCVDetectOpenCL.cmake b/cmake/OpenCVDetectOpenCL.cmake
index 100fb0e..ab30bf9 100644
--- a/cmake/OpenCVDetectOpenCL.cmake
+++ b/cmake/OpenCVDetectOpenCL.cmake
@@ -1,3 +1,8 @@
+find_package(OpenCLHeaders REQUIRED)
+if(NOT TARGET OpenCL::Headers)
+  message(FATAL_ERROR "OpenCL::Headers not found")
+endif()
+
 set(OPENCL_FOUND ON CACHE BOOL "OpenCL library is found")
 if(APPLE)
   set(OPENCL_LIBRARY "-framework OpenCL" CACHE STRING "OpenCL library")
@@ -5,6 +10,7 @@ if(APPLE)
 else()
   set(OPENCL_LIBRARY "" CACHE STRING "OpenCL library")
   set(OPENCL_INCLUDE_DIR "${OpenCV_SOURCE_DIR}/3rdparty/include/opencl/1.2" CACHE PATH "OpenCL include directory")
+  get_target_property(OPENCL_INCLUDE_DIR OpenCL::Headers INTERFACE_INCLUDE_DIRECTORIES)
 endif()
 mark_as_advanced(OPENCL_INCLUDE_DIR OPENCL_LIBRARY)