https://github.com/imageworks/OpenColorIO/pull/96

This adds a compile-time option to use boost ptr instead of tr1. The
tr1 ptr is defaut, but for folks who had already locked off on OCIO
0.7s ABI before the removal of boost (such a Imageworks), this lets us
compile without breaking binary compatibility. This code path will
also be necessary for windows support, which apparently does not have
tr1 support yet.

-- Jeremy