STAN_OPENCL implies STANCFLAGS=--use-opencl, avoid duplicates

[WardBrian]

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

See stan-dev/cmdstanr#1111 for more details. Essentially, it is the correct behavior for CmdStan's build system to set only STAN_OPENCL when the user requests opencl capabilities. This flag enables the C++ code and adds the proper flag to stanc3

Copyright and Licensing

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Simons Foundation

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• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)