Added initial OpenCL stub.

src/backend/backend.cmake

@@ -1,13 +1,16 @@
 include (src/backend/cpu/cpu.cmake)
+include (src/backend/opencl/opencl.cmake)
 include (src/backend/common/common.cmake)
 
 
 set(HEADERS_BACKEND
     "${HEADERS_BACKEND_COMMON}"
     "${HEADERS_BACKEND_CPU}"
+    "${HEADERS_BACKEND_OPENCL}"
    )
 
 set(SOURCES_BACKEND
     "${SOURCES_BACKEND_COMMON}"
     "${SOURCES_BACKEND_CPU}"
+    "${SOURCES_BACKEND_OPENCL}"
    )