Build against sytem dependancies if available

If CLHPP, ICD-Loader, and Headers are installed on the system, use
those instead of the submodules. This is beneficial when packaging
the SDK for distribution, as the submodules can be built as their
own seperate packages.

CMakeLists.txt

@@ -39,9 +39,20 @@ if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
     set(CMAKE_INSTALL_PREFIX "${PROJECT_SOURCE_DIR}/install" CACHE PATH "Install Path" FORCE)
 endif()
 
-add_subdirectory(external/OpenCL-Headers)
-add_subdirectory(external/OpenCL-ICD-Loader)
-add_subdirectory(external/OpenCL-CLHPP)
+find_package (OpenCLHeaders)
+if (NOT TARGET OpenCL::Headers)
+    add_subdirectory(external/OpenCL-Headers)
+endif ()
+
+find_package (OpenCLICDLoader)
+if (NOT TARGET OpenCL::OpenCL)
+    add_subdirectory(external/OpenCL-ICD-Loader)
+endif ()
+
+find_package (OpenCLHeadersCpp)
+if (NOT TARGET OpenCL::HeadersCpp)
+    add_subdirectory(external/OpenCL-CLHPP)
+endif ()
 
 list(APPEND CMAKE_MODULE_PATH
   ${PROJECT_SOURCE_DIR}/cmake