dev-libs/beignet: fix detection of OCL ICD and installation of ICD files

dev-libs/beignet: fix detection of OCL ICD and installation of ICD files

In the past, Beignet CMake scripts would automatically enable its compatibility
with the OCL ICD Loader depending on whether dev-libs/ocl-icd was found in the
system or not. Patch the scripts so that ocl-icd is only searched for if an
appropriate option is set, and define a local use flag which controls the value
of this option (and of course the dependency on dev-libs/ocl-icd). Nb. the
patch has been submitted upstream.

Moreover, given we always install Beignet ICD files (they are needed by
eselect-opencl) do not install another copy when OCL ICD Loader has been
enabled.

Gentoo-Bug: https://bugs.gentoo.org/show_bug.cgi?id=599512

Package-Manager: portage-2.3.2