graphics/mesa-devel: expose OpenCL support

Overrides lang/clover but still required for mesa.icd unless built
in standalone mode (DEFAULT_VERSIONS+=gl=mesa-devel). For example,
on Intel GPUs one can set IRIS_ENABLE_CLOVER=1 and RUSTICL_ENABLE=iris
via environ(7) to enable both Clover and Rusticl.

https://docs.mesa3d.org/envvars.html#rusticl-environment-variables
https://github.com/OCL-dev/ocl-icd/blob/master/doc/libOpenCL.7.txt.in