Added fixes for OSX public build.

Helped boost find the libraries it has just built - otherwise boost_wave fails to link because it can't find boost_system. Added notes about a modification to the Alembic CMake setup necessary to get the build to work - the Alembic tests don't compile on OS X so we must skip them. I tried setting the variable on the CMake command line but it had no effect - will get a patched version of Alembic into the dependencies tarball instead.
Merge pull request #649 from johnhaddon/osxPublicBuildFixes

Added fixes for OSX public build.