qemu-thread-posix: Fix build against older glibc version

pthread_setname_np was introduced with 2.12.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

util/qemu-thread-posix.c

@@ -420,7 +420,7 @@ void qemu_thread_create(QemuThread *thread, const char *name,
     if (err)
         error_exit(err, __func__);
 
-#ifdef _GNU_SOURCE
+#if defined(__GLIBC__) && (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 12))
     if (name_threads) {
         pthread_setname_np(thread->thread, name);
     }