fix compile errors in non-threadsafe build

src/BulletDynamics/Dynamics/btDiscreteDynamicsWorldMt.cpp

@@ -114,7 +114,10 @@ struct InplaceSolverIslandCallbackMt : public btSimulationIslandManagerMt::Islan
 
 btConstraintSolverPoolMt::ThreadSolver* btConstraintSolverPoolMt::getAndLockThreadSolver()
 {
-    int i = btGetCurrentThreadIndex() % m_solvers.size();
+    int i = 0;
+#if BT_THREADSAFE
+    i = btGetCurrentThreadIndex() % m_solvers.size();
+#endif // #if BT_THREADSAFE
     while ( true )
     {
         ThreadSolver& solver = m_solvers[ i ];

src/LinearMath/btThreads.cpp

@@ -465,6 +465,12 @@ bool btSpinMutex::tryLock()
     return true;
 }
 
+// non-parallel version of btParallelFor
+void btParallelFor( int iBegin, int iEnd, int grainSize, const btIParallelForBody& body )
+{
+    btAssert(!"called btParallelFor in non-threadsafe build. enable BT_THREADSAFE");
+    body.forLoop( iBegin, iEnd );
+}
 
 #endif // #if BT_THREADSAFE