provide some backwards compatibility with existing binaries

[kevinushey]

Mainly to avoid breakage on CRAN, and to allow existing package binaries to continue to work. (At least, some subset of them.)

[MaxAtoms]

Thank you! The missing binary compatibility broke our pipeline because we explicitly requested 5.1.9, but the package lamW was built against 5.1.10 on the Posit Public Package Manager:
https://forum.posit.co/t/discrepancy-between-snapshots/197445. Glad to see that this should not happen in the future

Edit: Looking over the changelog, I am actually not sure whether this PR is related and would have helped

[kevinushey]

Yeah, that's right -- this PR is actually probably responsible for the breakage you saw. In particular, packages built against RcppParallel 5.1.10 won't work if RcppParallel 5.1.9 is currently installed.

This change was ultimately requested by CRAN, and is necessary to provide RcppParallel a pathway to updating the bundled version of TBB without breaking all existing CRAN binary packages.