Include <threads.h> if possible to get thread_local definition

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>

src/basic/macro.h

@@ -297,20 +297,6 @@ static inline int __coverity_check_and_return__(int condition) {
              p != (typeof(p)) POINTER_MAX;                                               \
              p = *(++_l))
 
-/* Define C11 thread_local attribute even on older gcc compiler
- * version */
-#ifndef thread_local
-/*
- * Don't break on glibc < 2.16 that doesn't define __STDC_NO_THREADS__
- * see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53769
- */
-#if __STDC_VERSION__ >= 201112L && !(defined(__STDC_NO_THREADS__) || (defined(__GNU_LIBRARY__) && __GLIBC__ == 2 && __GLIBC_MINOR__ < 16))
-#define thread_local _Thread_local
-#else
-#define thread_local __thread
-#endif
-#endif
-
 #define DEFINE_TRIVIAL_DESTRUCTOR(name, type, func)             \
         static inline void name(type *p) {                      \
                 func(p);                                        \