custom bullet header path and library names
Please allow for a custom bullet header path and library names
In gentoo the headers are under /usr/include/bullet/ (e.g.: /usr/include/bullet/BulletCollision/CollisionShapes/btCollisionShape.h) and the libraries are named libBulletCollision.so
last there isn't a libbulletmath.so or libBulletMath.so
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