Define STRICT_R_HEADERS in Makevars*, use M_PI in two files

[eddelbuettel]

Dear Kosuke, dear emIRT team,

Your CRAN package emIRT uses Rcpp, and is affected if we add a definition of STRICT_R_HEADERS as we would like to do. Please see the discussion at
RcppCore/Rcpp#1158
and the links therein for more context on this.

Here, instead of prefixing each #include <Rcpp.h> with STRICT_R_HEADERS I changed src/Makevars* to define it which is less invasive still ensures we will not get surprised later. The actual change that is needed is the change from PI to M_PI in two files. (And while looking at src/Makevars* I removed the old link instruction for Rcpp we have not needed anymore since around 2013.)

It would be lovely if you could apply this. There is no strong urgency: we aim to get this done over all affected packages in the space of a few months. If you apply it, would you mind dropping me a note by email or swinging by
RcppCore/Rcpp#1158
to confirm?

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.