SConstruct : Fix OSX linking errors.
SConstruct : Fix OSX linking errors.

This fixes the `ld: library not found for -lGafferImageBindings` build errors that have plagued folks for a while (see #1700 for example). Had to take a little trip into the internals of SCons to get to the bottom of this one.

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