SMTK: enable build by default.
This MR enables a standalone SMTK build by default, reverting the default build configuration to be the same as it was before the reintroduction of a thirdparty SMTK.
This MR enables a standalone SMTK build by default, reverting the default build configuration to be the same as it was before the reintroduction of a thirdparty SMTK.