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/capability-util.c

@@ -14,6 +14,7 @@
 #include "logarithm.h"
 #include "macro.h"
 #include "missing_prctl.h"
+#include "missing_threads.h"
 #include "parse-util.h"
 #include "user-util.h"