https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53769
--- Comment #16 from Jonathan Wakely <redi at gcc dot gnu.org> --- As explained above, this is not something that can be fixed in GCC. The macro depends on whether or not your C library provides <threads.h> which GCC doesn't know. It's a problem with your C library, which is unlikely to be fixed if you're using an OS from before 2011.
