I also trying to build GLOW with
cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DGLOW_WITH_CPU=1 -DGLOW_WITH_OPENCL=1 ..

But recently have some problems
CMake Error at lib/Backends/OpenCL/CMakeLists.txt:32 (add_library):
  Target "OpenCLBackend" links to target "OpenCL::OpenCL" but the target was
  not found.  Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?


That I supposed to do?
I believe that problem come from
target_link_libraries(OpenCLBackend
                      PUBLIC
                      OpenCL::OpenCL)


from
lib/Backends/OpenCL/CMakeLists.txt

find_package finds OpenCL successfully (no error messages about that)
That is  “OpenCL::OpenCL” ?
UPDATE
This issue solved by using more recent cmake, cmake 3.5.2 DID NOT WORK please update cmake minimal required