[libcxx-commits] [libcxx] [libc++] Gate _BitInt library extensions behind opt-in macro per P3666R4 (PR #203841)

[Xavier Roche via libcxx-commits]

xroche wrote:

Note to reviewers: This is a proof-of-concept, not an endorsement of the mechanism itself :smile_cat: 

https://github.com/llvm/llvm-project/pull/203841