[BUG] Compilation error of unified on opencl

Compilation error:

cmake  -DAF_BUILD_OPENCL=OFF ..

...

-- Could NOT find OpenCL (missing: OpenCL_LIBRARY OpenCL_INCLUDE_DIR) 

-- Could NOT find OpenCL (missing: OpenCL_LIBRARY OpenCL_INCLUDE_DIR) (Required is at least version "1.2")

...

-- Configuring done

-- Generating done

-- Build files have been written to: /home/wtambellini/repos/arrayfire/release

...

make

...

In file included from /home/wtambellini/repos/arrayfire/src/api/unified/opencl.cpp:13:0:

/home/wtambellini/repos/arrayfire/include/af/opencl.h:14:19: fatal error: CL/cl.h: No such file or directory

 #include <CL/cl.h>

                   ^

compilation terminated.



Description

===========

Did you build ArrayFire yourself or did you use the official installers

myself

Which backend is experiencing this issue? (CPU, CUDA, OpenCL)

unified

Do you have a workaround?

no

Can the bug be reproduced reliably on your system?

yes

A clear and concise description of what you expected to happen.

compilation of unified



Reproducible Code and/or Steps

------------------------------

Try to build AF on a linux machine wihtout any opencl header installed 



No surprise: the unified lib tries to build  src/api/unified/opencl.cpp even if opencl not found.

Any way to skip src/api/unified/opencl.cpp if no opencl found ?



System Information

------------------

1. ArrayFire version: today master

2. Devices installed on the system: Quadro RTX 3000

3. (optional) Output from the af::info() function if applicable.

4. Output from the following scripts:



lsb_release -a

No LSB modules are available.

Distributor ID:	Ubuntu

Description:	Ubuntu 18.04 LTS (whitehaven-mlk X84)

Release:	18.04

Codename:	bionic



name, memory.total [MiB], driver_version

Quadro RTX 3000, 5934 MiB, 440.33.01



clinfo not found.



Checklist

---------

- [X ] Using the latest available ArrayFire release

- [X ] GPU drivers are up to date

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