ansi-c: introduce conditional_keyword(c, t)

[kroening]

This introduces a helper function in the C scanner to deal with a larger number of cases to enable or disable keywords of the various C/C++ dialects.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Attention: Patch coverage is 48.78049% with 42 lines in your changes missing coverage. Please review.

Project coverage is 80.36%. Comparing base (10a14c9) to head (ce7029f).
Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/ansi-c/scanner.l	48.78%	42 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8621      +/-   ##
===========================================
- Coverage    80.38%   80.36%   -0.02%     
===========================================
  Files         1686     1686              
  Lines       206772   206799      +27     
  Branches        85       84       -1     
===========================================
- Hits        166213   166200      -13     
- Misses       40559    40599      +40     

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