Fix GitHub Actions for threading.

PiperOrigin-RevId: 922926969
Change-Id: I968e6b130b6125dbc0ec65ba913d04292270b563

src/engine/engine_thread.cc

@@ -24,6 +24,10 @@
 #include <mujoco/mjmodel.h>
 #include "engine/engine_memory.h"
 
+#if defined(__cpp_lib_atomic_wait)
+
+// ----------------------------- multithreaded implementation --------------------------------------
+
 // context for thread pool stored on mjData
 class ThreadPoolContext {
  public:
@@ -129,7 +133,6 @@ class ThreadPoolContext {
 };
 
 
-
 // create a thread pool with nthread threads
 void mju_threadpool(mjData* d, int nthread) {
   if (d->threadpool) {
@@ -188,3 +191,23 @@ int mju_numThread(const mjData* d) {
   ThreadPoolContext* ctx = reinterpret_cast<ThreadPoolContext*>(d->threadpool);
   return ctx ? ctx->ThreadCount() + 1 : 1;
 }
+
+#else
+
+// -------------------------------------- fallback  ------------------------------------------------
+
+void mju_threadpool(mjData* d, int nthread) {
+  // No-op
+}
+
+void mju_dispatch(const mjModel* m, mjData* d, mjTaskFunc func, void* arg, int ntask) {
+  for (int i = 0; i < ntask; i++) {
+    func(m, d, arg, 0, i);
+  }
+}
+
+int mju_numThread(const mjData* d) {
+  return 1;
+}
+
+#endif  // __cpp_lib_atomic_wait