Theorically we could define __STDC_NO_THREADS__ internally in GCC, but
threads.h probably will be added in the future, and the compiler won't
notice.
Using this stdc-predef.h header and including it in GCC would solve this
nicely, and saves us from hardcoding.
The header can eventually be removed when obsolete.
Change-Id: I29aa58686e3c45449dc63e02e5a9e13a960b9090
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4097
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>
