[clang] [clang][PP] Add extension to predefine target OS macros (PR #74676)

[via cfe-commits]

stbergmann wrote:

Is the motivation for this change in behavior (on macOS) documented somewhere?  Besides zlib as already discussed above, pre-defining `TARGET_OS_MAC` apparently at least also affects some libpng builds (and I ultimately had to globally set `-fno-define-target-os-macros` for now to make a test build of LibreOffice and all its included external libraries work).

https://github.com/llvm/llvm-project/pull/74676