[llvm] [libc++] Use private CMake flags to enable the pragma system_header macro when building (PR #138826)

[Valery Mironov via llvm-commits]

MBkkt wrote:

> Ok, well as I said I don't mind this patch, but be aware that this consumption method for the library is not considered supported.

@ldionne Ok, thanks, but is there any other way to build libc++ from source in CMake project (except writing own cmake for libc++?)?

https://github.com/llvm/llvm-project/pull/138826