Make get_cmdstan_flags() robust to noisy make output

[jgabry]

Submission Checklist

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

Summary

Fixes #1162

Parse make print-<FLAG> output line-by-line so cmdstanr ignores unrelated make chatter such as directory-entering messages triggered by MAKEFLAGS=-w. This fixes a failure where get_cmdstan_flags("STANCFLAGS") could return junk tokens, which then caused cmdstan_model() to error when checking local stanc flags.

Also:

• return character(0) for empty STANCFLAGS
• use a length-safe check when combining local stanc flags

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 (codex assisted with debugging, writing test code, and writing the PR summary)

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

❌ Patch coverage is 81.81818% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.85%. Comparing base (418b1c5) to head (8b92b81).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
R/utils.R	83.33%	5 Missing ⚠️
R/cpp_opts.R	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1163      +/-   ##
==========================================
- Coverage   90.85%   90.85%   -0.01%     
==========================================
  Files          14       14              
  Lines        5908     5924      +16     
==========================================
+ Hits         5368     5382      +14     
- Misses        540      542       +2     

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