IN C23, thread_local is a reserved keyword and we shall therefore do nothing to redefine it. glibc has it defined for older standard version with the right conditions. v2 by Yu Watanabe: Move the definition to missing_threads.h like the way we define e.g. missing syscalls or missing definitions, and include it by the users. Co-authored-by: Yu Watanabe <watanabe.yu+github@gmail.com>