add the BT_ prefix for all #ifdef guards, to avoid conflicts with developers own header defines.

Fixes Issue 497

//license header
#ifndef BT_RIGIDBODY_H
#define BT_RIGIDBODY_H
class btRigidBody
{
};
#endif //BT_RIGIDBODY_H

src/LinearMath/btDefaultMotionState.h

@@ -1,5 +1,5 @@
-#ifndef DEFAULT_MOTION_STATE_H
-#define DEFAULT_MOTION_STATE_H
+#ifndef BT_DEFAULT_MOTION_STATE_H
+#define BT_DEFAULT_MOTION_STATE_H
 
 #include "btMotionState.h"
 
@@ -37,4 +37,4 @@ struct	btDefaultMotionState : public btMotionState
 
 };
 
-#endif //DEFAULT_MOTION_STATE_H
+#endif //BT_DEFAULT_MOTION_STATE_H