[Bug c/69982] missing warning when using __thread with -std=c99 -pedantic

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69982

--- Comment #1 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
Note: This actually means that a program that switches from __thread to
_Thread_local (which could be regarded as more portable) could suddenly fail to
build with "-std=c99 -pedantic -Werror".