C11 uses _Thread_local as the keyword, which thread_local macro
is defined to (inside of <threads.h>, which mpv doesn't use):
https://port70.net/~nsz/c/c11/n1570.html#7.26.1p3
C23 promotes thread_local to a keyword and marks _Thread_local
as obsolete and to be avoided in new code:
https://www.iso-9899.info/n3220.html#6.4.1p3
so just use thread_local directly, which is future-proof while
having a fallback define in osdep/threads.h for pre-C23 cases.
NRK
b09e06a920