Include CmdStan CPPFLAGS when exposing functions

[andrjohns]

Submission Checklist

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

Summary

From issue on the forums: some flags in make/local (e.g., TBB_INTERFACE_NEW, etc.) weren't being passed to Rcpp when exposing functions and model methods

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):
Andrew Johnson

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/)

[jgabry]

Looks good!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.89%. Comparing base (b271b7b) to head (8cfd4a5).
Report is 24 commits behind head on master.

❗ Current head 8cfd4a5 differs from pull request most recent head 7c55d5d

Please upload reports for the commit 7c55d5d to get more accurate results.

Files	Patch %	Lines
R/utils.R	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1003      +/-   ##
==========================================
+ Coverage   84.20%   85.89%   +1.68%     
==========================================
  Files          13       13              
  Lines        4793     4800       +7     
==========================================
+ Hits         4036     4123      +87     
+ Misses        757      677      -80     

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