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: https://github.com/stan-dev/rstanarm/issues/601

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[bgoodri]

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