Use message() to signal very old helper functions are redundant (closes #1412)

[eddelbuettel]

This simple PR addresses the issue raised in #1412 about the long-redundant helper functions that were prior to 2013 (!!) to constrct working src/Makevars* but have not been needed since.

A reverse-dependency check has been started. I will keep it as 'draft' until that is done (likely Monday).

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 addresses issue #1412 by adding informative deprecation messages to six helper functions (RcppLdPath, RcppLdFlags, CxxFlags, LdFlags, RcppCxx0xFlags, and Cxx0xFlags) that have been redundant since 2013. The messages use message() to gently notify users that these functions may be removed in 2027, with a recommendation to update their Makevars files.

Key Changes:

• Added consistent deprecation messages to six long-redundant helper functions using message()
• Updated ChangeLog to document the changes

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
R/RcppLdpath.R	Added message() calls to six deprecated helper functions to inform users of their redundancy and potential future removal
ChangeLog	Documented the addition of deprecation warnings via message() for the affected functions

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[eddelbuettel]

This is ready for review as well now as the reverse-dependency run passed without any issues.

[Enchufa2]

LGTM!