Switch PI to M_PI to support STRICT_R_HEADERS

[eddelbuettel]

Dear Johan, dear eulerr team,

The Rcpp team is trying to move towards defining STRICT_R_HEADERS by default. Please the issue ticket at RcppCore/Rcpp#1158 for motivation and history.

Your package uses PI (instead of the standard C define M_PI, or the newer Armadillo constant arma::datum::pi) and PI goes away when we set STRICT_R_HEADERS (as a #define in a header or source file, a -DSTRICT_R_HEADERS as a compiler flag, or as a #define in the Rcpp sources as we currently do). We plan to enable STRICT_R_HEADERS by the Jan 2022 release of Rcpp, and will likely offer you a define to suppress it. So if you really do not want the change you can prevent it -- see these lines in Rcpp for details:
https://github.com/RcppCore/Rcpp/blob/e79c70e76bc2a776d2d57287f7192dbdbcb292aa/inst/include/Rcpp/r/headers.h#L28-L38

As discussed in RcppCore/Rcpp#1158, this is not urgent, but we of course welcome relatively prompt resolution at CRAN so when we continue to test for this (at a likely montly pace) so we do not get false positives as we will continue to use the CRAN set of packages (as opposed to hand-curated set of upstream dev versions).

Many thanks for your help, and I hope you continue to find Rcpp helpful. Please don't hesitate to ask if you have any questions.

[eddelbuettel]

Thanks for the prompt merge!

[jolars]

Thanks for the pull request!