[clang] Revert "[Fix] Disable fdefine-target-os-macros for now" (PR #78353)

[Zixu Wang via cfe-commits]

zixu-w wrote:


Hi @zmodem !
> Could you add a release note about this, explaining how it changes the macros and what actions users should take?

That's a good idea, I should be able to put something up.
> 
> We've already seen that it broke zlib builds, and we're having issues with libpng too in Chromium (https://crbug.com/1519899), so users are likely to run into this.

FWIW, we've also noticed the issue and upstreamed a fix for libpng (https://github.com/pnggroup/libpng/pull/529)

https://github.com/llvm/llvm-project/pull/78353