Emit message if R.h included, can turn to warning

[eddelbuettel]

This addresses #1410 and adds a message if R.h (or one of three other headers) is seen before Rcpp.h is included.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Preferably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[Copilot]

Pull request overview

This PR adds a compile-time check to detect when R headers (R.h, Rinterface.h, Rinternals.h, or Rdefines.h) are included before Rcpp headers, which can cause hard-to-debug errors. The check emits a pragma message directing users to issue #1410 for more information, and provides an escape hatch via the RCPP_NO_R_HEADERS_CHECK macro.

• New header file check_r_headers.h that detects pre-included R headers
• Integration of the check into RcppCommon.h as the first include
• Copyright year updates for 2025

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
inst/include/Rcpp/r/check_r_headers.h	New header file that checks for R headers included before Rcpp and emits pragma messages when detected
inst/include/RcppCommon.h	Includes the new check_r_headers.h file and updates copyright year to 2025
ChangeLog	Documents the addition of the new header check feature and the RCPP_NO_R_HEADERS_CHECK escape hatch

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[Enchufa2]

Good robot. :) LGTM!

[eddelbuettel]

Yes, as uncool it is to admit it, I now turn them on all the time. It is especially good at the nitpicky stuff.

[eddelbuettel]

Ok, having waited the polite 24 hours for veto or comment (hi @kevinushey we know you're busy so no worries) I will now fold this in.