[Beignet] [PATCH v2] Enable OpenCL 2.0 only where supported

[Rebecca N. Palmer]

> regarding whether OpenCL 2.0
> is compiled or not I would rather think of something like that:
> 
> – if llvm39 and libdrm2466 are found, compile with OpenCL 2.0 by default.
> – if they’re not, compile without it. Issue an error message if
> -DENABLE_OPENCL_20=1 was passed.

Agreed - we actually do that in Debian, but our current method
(https://anonscm.debian.org/cgit/pkg-opencl/beignet.git/tree/debian/rules#n16)
involves Debian-specific tools.