Workaround for macos-14

[jeroen]

Until Simon updates the boost in his bundle.

This code path is only taken on r-universe so should not upset CRAN.

[eddelbuettel]

Agreed on codepath, we can keep it along with that fixed path for the 1.31.1 tarball you slurp in.

Two tiny requests though:

• Can you add a comment next the updated PKG_CXXFLAGS to mark the clang version that required this?
• Can you please, as usual, add a quick ChangeLog entry?

Thanks!