Wrapper : Tolerate external python builds.
Wrapper : Tolerate external python builds.

This is necessary to allow the documentation to build at Image Engine, where Gaffer is built against external dependencies rather than against the gafferDependencies package.

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