Use OpenCL::OpenCL interface library

[blowekamp]

Use imported OpenCL targets from CMake

Replace variables with imported target for OpenCL usage.

Use the modern OpenCL::OpenCL imported interface target from
CMake. This dependecy is expressed in the public interface of
ITKGPUCommon, and does not need to be repeated in modules which use
ITKGPUCommon.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)
• Added test (or behavior not changed)
• Updated API documentation (or API not changed)
• Added license to new files (if any)
• Added Python wrapping to new files (if any) as described in ITK Software Guide Section 9.5
• Added ITK examples for all new major features (if any)

Refer to the ITK Software Guide for
further development details if necessary.

[dzenanz]

Sounds great!