Here's the reason for anyone wondering:
_Thread_local is reserved in C99, so we don't need a -pedantic warning.
(Quoting the standard: "All identifiers that begin with an underscore and
either an uppercase letter or another underscore are always reserved for any
use.")
Which I don't really understand.. I mean, you don't need to warn about anything, that's what "warning" means, and having no way of warning that I'm using things from C11 seems weird to me, given how many other warnings exist.