fix some warnings, disable gimpact by default in world importer,

use DIRECTLY_UPDATE_VELOCITY_DURING_SOLVER_ITERATIONS by default for now,
until we find the issue with some failing test cases in btMultiBody
fix a crashing issue in MyMultiBodyCreator.cpp (uninitialized variable)
disable excessive debug printf in URDF2Bullet

Extras/Serialize/BulletWorldImporter/btBulletWorldImporter.cpp

@@ -18,8 +18,9 @@ subject to the following restrictions:
 #include "../BulletFileLoader/btBulletFile.h"
 
 #include "btBulletDynamicsCommon.h"
+#ifndef USE_GIMPACT
 #include "BulletCollision/Gimpact/btGImpactShape.h"
-
+#endif
 
 
 //#define USE_INTERNAL_EDGE_UTILITY