[libcxx-commits] [libcxx] [libc++] Enable availability based on the compiler instead of __has_extension (PR #84065)

[Aaron Ballman via libcxx-commits]

AaronBallman wrote:

> Given that it's just been added in GCC trunk (I'm quite surprised that got in) and the [documentation](https://gcc.gnu.org/onlinedocs/cpp/_005f_005fhas_005fextension.html) says that it's for compatibility with clang, this isn't really surprising.

I was surprised at how recently it was added to GCC, but what did surprise me was that the behavior was consistent between `-pedantic-errors` and `-pedantic -Werror`. Regardless, if we change behavior in Clang, we should be sure to coordinate with the GCC developers so they're aware of the changes.

https://github.com/llvm/llvm-project/pull/84065