Fix compilation failures for Stan models with `#include` statements

[andrjohns]

Submission Checklist

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

Summary

Compilation can fail for some systems when looking for include paths, as the quoting of arguments is handled differently when being passed to make as opposed to directly to stanc

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/)