Download the Intel OpenCL runtime & libraries (you don’t need the sdk).
install those RPMs. They will install to /opt/intel/opencl

cd gr-fosphor/build

here is the key step:

cmake -DOpenCL_LIBRARY=/opt/intel/opencl/libOpenCL.so -DOpenCL_INCLUDE_DIR=/opt/intel/opencl/include ..

then just carry on with the normal instructions:

make
sudo make install
sudo ldconfig