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/btRandom.h

@@ -14,8 +14,8 @@ subject to the following restrictions:
 
 
 
-#ifndef GEN_RANDOM_H
-#define GEN_RANDOM_H
+#ifndef BT_GEN_RANDOM_H
+#define BT_GEN_RANDOM_H
 
 #ifdef MT19937
 
@@ -38,5 +38,5 @@ SIMD_FORCE_INLINE unsigned int GEN_rand()                   { return rand(); }
 
 #endif
 
-#endif
+#endif //BT_GEN_RANDOM_H