Update expose_stan_functions flags for c++17

[andrjohns]

Summary:

expose_stan_functions fails under c++17 as the Rcpp plugin is missing the updated Boost flags. This is causing rstanarm test failures on CRAN: stan-dev/rstanarm#601

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: GPLv3 (https://opensource.org/licenses/GPL-3.0)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[bgoodri]

This is needed, but I'm going to bump all of the C++14 stuff in rstan to C++17.