I know this thread is a bit old, but it's worth mentioning that CL/GL sharing being enabled causes fosphor to be broken altogether on my setup as of the latest master commit (possibly earlier). Everything builds, but the following runtime error occurs:
[!] CL Error (-5, /home/user/gr-fosphor/lib/fosphor/cl.c:480): Unable to share spectrum VBO into OpenCL context
As previously mentioned, everything works okay if I change the value of FLG_FOSPHOR_USE_CLGL_SHARING in private.h to 0.
Sylvain, do you always have the value of this flag as zero in your building and testing? Or do you leave it as 1 and fosphor works because you're using Intel CPU OpenCL or something? Maybe check in "#define FLG_FOSPHOR_USE_CLGL_SHARING (0<<0)" into the source repo until you can verify that this code path works correctly?
Details of my setup:
x86_64 (Xeon D)
Ubuntu 16.04
NVIDIA Titan Xp
NVIDIA Driver 375.66 from the standard Ubuntu (xenial-updates)
nvidia-opencl-icl-375 from the standard Ubuntu repos
I'm happy to test things out and will do some of my own debugging.