Use the platform OpenCL library on Apple.

[maleadt]

Fixes #205

I'm not inclined to merge this, though. Apple has deprecated OpenCL, only supporting v1.2, which is ancient. I'll leave this up in case somebody really needs this, and wants to finish this. That would either involve making everything compatible with OpenCL v1.2, or introducing an opt-in preference to use the system OpenCL platform for backwards compatibility.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 53.33%. Comparing base (9ccf867) to head (d766d80).

Files with missing lines	Patch %	Lines
src/util.jl	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #221       +/-   ##
===========================================
- Coverage   70.45%   53.33%   -17.13%     
===========================================
  Files           3        3               
  Lines         132      135        +3     
===========================================
- Hits           93       72       -21     
- Misses         39       63       +24     

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