Exclude STAN_VERSION from cpp_options()

[jgabry]

Submission Checklist

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

Summary

model_compile_info() returns uppercase keys, so the comparison against lowercase "stan_version" never matched and the executable's Stan version was stored as if it were a C++ option. We can just use tolower() to fix it.

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 91.69%. Comparing base (5f6e8c3) to head (48a07b9).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1215   +/-   ##
=======================================
  Coverage   91.69%   91.69%           
=======================================
  Files          15       15           
  Lines        6246     6246           
=======================================
  Hits         5727     5727           
  Misses        519      519           

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