Intel OpenCL not recognized

c.f. https://github.com/cdeterman/gpuR/issues/109 and https://github.com/s-u/OpenCL/issues/4
``` r
RcppArrayFire::arrayfire_info()
#> ArrayFire v3.5.1 (OpenCL, 64-bit Linux, build 0a675e8)
#> [0] BEIGNET: Intel(R) HD Graphics Skylake ULT GT2, 4096 MB
#> -1- POCL: pthread-Intel(R) Core(TM) i7-6600U CPU @ 2.60GHz, 17517 MB
```