Version-guard STRING_PTR definition

[MichaelChirico]

Part of #6180. I'm not quite sure why the #ifndef approach failed us, but I confirmed this works, as well as confirming R 3.5.0 is the first version where STRING_PTR_RO() is in a release:

https://github.com/wch/r-source/blob/tags/R-3-4-4/src/include/Rinternals.h
https://github.com/wch/r-source/blob/tags/R-3-5-0/src/include/Rinternals.h

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.61%. Comparing base (436bd6c) to head (1c6ff24).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6654   +/-   ##
=======================================
  Coverage   98.61%   98.61%           
=======================================
  Files          79       79           
  Lines       14558    14558           
=======================================
  Hits        14357    14357           
  Misses        201      201           

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[github-actions]

Generated via commit 1c6ff24

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	4 minutes and 50 seconds
Installing different package versions	7 minutes and 37 seconds
Running and plotting the test cases	2 minutes and 14 seconds

[aitap]

Good catch! STRING_PTR_RO used to be available as a macro with USE_RINTERNALS defined, otherwise as the exported function-wrapper with slightly better safety. Most of USE_RINTERNALS disappeared in R-4.2.0.

[MichaelChirico]

Ah, well that makes sense! #ifndef doesn't work because simply it's not a macro! 🤪