define STRICT_R_HEADERS, include float.h, adjust dbl eps constant

[eddelbuettel]

Dear Maogui,

As you know from yesterday's email (to which you already responded -- much appreciated), your CRAN package atakrig uses Rcpp, and uses in (just one file) definitions of DOUBLE_EPS that would go away if we enabled STRICT_R_HEADERS -- as we would like to. Please see the discussion at RcppCore/Rcpp#898 RcppCore/Rcpp#1158 and the links therein for more context.

By simply including the standard header <float.h> and defining STRICT_R_HEADERS we can switch to DBL_EPSILON as the email patch and, for easier use, PR here show.

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#898 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.

Edit 2021-04-29: Link to 1158 instead of 989