Added fixes for OSX public build.
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.

andrewkaufman/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,hradec/gaffer,cedriclaunay/gaffer,cedriclaunay/gaffer,davidsminor/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,davidsminor/gaffer,ivanimanishi/gaffer,boberfly/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,hradec/gaffer,davidsminor/gaffer,chippey/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,davidsminor/gaffer,boberfly/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,ivanimanishi/gaffer,cedriclaunay/gaffer,boberfly/gaffer,hradec/gaffer,hradec/gaffer,goddardl/gaffer,andrewkaufman/gaffer,chippey/gaffer,lucienfostier/gaffer,goddardl/gaffer,ivanimanishi/gaffer,johnhaddon/gaffer,goddardl/gaffer,ivanimanishi/gaffer,hradec/gaffer,chippey/gaffer,boberfly/gaffer,lucienfostier/gaffer,chippey/gaffer,goddardl/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,ImageEngine/gaffer,cedriclaunay/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,appleseedhq/gaffer