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/btTransformUtil.h

@@ -13,8 +13,8 @@ subject to the following restrictions:
 */
 
 
-#ifndef SIMD_TRANSFORM_UTIL_H
-#define SIMD_TRANSFORM_UTIL_H
+#ifndef BT_TRANSFORM_UTIL_H
+#define BT_TRANSFORM_UTIL_H
 
 #include "btTransform.h"
 #define ANGULAR_MOTION_THRESHOLD btScalar(0.5)*SIMD_HALF_PI
@@ -224,5 +224,5 @@ public:
 };
 
 
-#endif //SIMD_TRANSFORM_UTIL_H
+#endif //BT_TRANSFORM_UTIL_H