[Beignet] Building beignet with OpenCL 2.0 support in distros

[Rebecca N. Palmer]

As for the "OpenCL 2.0 = worse performance" problem, the OpenCL spec
actually _requires_ 2.0 compilers to default to 1.2, and only enable
2.0 if explicitly asked to:
https://www.khronos.org/registry/OpenCL/specs/opencl-2.0.pdf page 203

beignet currently defaults to 2.0 if it is enabled, but this is easy
to change: the default is set at backend/src/backend/program.cpp:1026