enhancement to current cpp option code

[katrinabrock]

Submission Checklist

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

Summary

• small change to support Improve Handling of cpp_options #1022
• just a refactor, no functional change

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):
Max Planck Institute of Animal Behavior

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/)

[katrinabrock]

:-/ I think maybe this actually can't be merged independently....will dig deeper later and either fix or close.