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/BulletCollision/CollisionDispatch/btCollisionConfiguration.h

@@ -15,6 +15,7 @@ subject to the following restrictions:
 
 #ifndef BT_COLLISION_CONFIGURATION
 #define BT_COLLISION_CONFIGURATION
+
 struct btCollisionAlgorithmCreateFunc;
 
 class btStackAlloc;