Snippets for m4 1.4.21 from Fedora s390x

Sourced from https://kojipkgs.fedoraproject.org/packages/m4/1.4.21/1.fc45/data/logs/s390x/build.log.

• test-thread_create.c:30:20: warning: 'worker_thread' defined but not used [-Wunused-variable]
     30 | static gl_thread_t worker_thread;
        |                    ^~~~~~~~~~~~~