Add THREADS_ENABLED macro in order to compile Godot to run on the main thread

core/os/condition_variable.h

@@ -33,6 +33,8 @@
 
 #include "core/os/mutex.h"
 
+#ifdef THREADS_ENABLED
+
 #ifdef MINGW_ENABLED
 #define MINGW_STDTHREAD_REDUNDANCY_WARNING
 #include "thirdparty/mingw-std-threads/mingw.condition_variable.h"
@@ -66,4 +68,16 @@ public:
 	}
 };
 
+#else // No threads.
+
+class ConditionVariable {
+public:
+	template <class BinaryMutexT>
+	void wait(const MutexLock<BinaryMutexT> &p_lock) const {}
+	void notify_one() const {}
+	void notify_all() const {}
+};
+
+#endif // THREADS_ENABLED
+
 #endif // CONDITION_VARIABLE_H