Add flag for stanc3 compatibility

[andrjohns]

Hi,

This PR adds a compiler flag to your Makevars files which is needed for compatibility with the upcoming version of rstan: -DUSE_STANC3

The Stan-to-c++ transpiler (stanc) was refactored after 2.21, and so different c++ is used in the new rstan package (>= 2.26). Because of this, the flag -DUSE_STANC3 needs to be added when a stan model is being run using the new rstan package (and the new stanc implementation).

This PR adds the compiler flag, which is conditionally set based on the installed rstan version.

Feel free to let me know if you need any more info.

Thanks!
Andrew

[helske]

Thanks, Andrew, is this something that will be handled by the rstantools::rstan_config() automatically in future versions of rstantools? As suggested by @bgoodri in #15, I now try to avoid modifying the Makevars directly (walker needs additional LAPACK/BLAS/FLIBS flags to work as it contains Armadillo C++ code as well), which I have now tried to circumvent by modifying the configure script instead. I guess this change could go there as well, but if it is soon handled automatically it probably doesn't matter.

[andrjohns]

Yep, this will be automatically handled by rstantools in the future. This PR is just an intermediate fix to work towards getting RStan CRAN-compatible