recognize thread and OpenCL support regardless of the capitalization

[jgabry]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Respect STAN_THREADS and STAN_OPENCL regardless of capitalization and be more consistent in documentation of cpp options

• fixes change cpp_options[["stan_threads"]] to cpp_options[["STAN_THREADS"]] #1100
• fixes cpp_options = list(stan_threads = TRUE) does not work when included in src/install.libs.R. wlandau/instantiate#30 and `cpp_options = list(stan_threads = TRUE)` does not work when included in `src/install.libs.R`. wlandau/instantiate#31
• advances STAN_THREADS in make/local not respected due to capitalisation conflict #765 (by recognizing mixed-case executable metadata for precompiled models. But metadata can still be lost after a later $compile() call, so leaving STAN_THREADS in make/local not respected due to capitalisation conflict #765 open).

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Jonah Gabry

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.76%. Comparing base (aae24ed) to head (436421d).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1212      +/-   ##
==========================================
+ Coverage   91.69%   91.76%   +0.07%     
==========================================
  Files          15       15              
  Lines        6246     6253       +7     
==========================================
+ Hits         5727     5738      +11     
+ Misses        519      515       -4     

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