[libc-commits] [libc] [libc] Use global preprocessor definitions for libc tuning (PR #71938)

[Guillaume Chatelet via libc-commits]

gchatelet wrote:

One caveat is that the configuration is global and leaks in the tests.

For instance, [setting the flag in a test instance](https://github.com/llvm/llvm-project/blob/601e8fdd28e50bdd8ccac24ec2a3e330b3651937/libc/src/string/CMakeLists.txt#L662) is useless if the configuration for the build bot already has the option set to `true`. Conversely, we cannot really test disabling the option (TBH I'm not sure how `add_compile_definitions` takes precedence over options passed through `add_entrypoint_object::COMPILE_OPTIONS`).

https://github.com/llvm/llvm-project/pull/71938