This macro is internal to libc++ and was removed by
https://github.com/llvm/llvm-project/commit/c6f3b7bcd0596d30f8dabecdfb9e44f9a07b6e4c

Abseil clients lacking this support should send a PR to add a
supported platform specific condition to the preprocessor to disable
it.

https://github.com/llvm/llvm-project/pull/89178#issuecomment-2075009955
suggests the feature test macros like __cpp_lib_filesystem now work.

PiperOrigin-RevId: 695454966
Change-Id: I9d4f710f422b9a2351d2780c219e8804f4162c07