COMP: Enable superbuild for Slicer4 extension.

JsonCpp and ParameterSerializer will be built in superbuild directory while Slicer provides everything else.

Change-Id: Id4be148c89c864cb75dcb2ff6451ad993e0a58ce
Merge pull request #166 from chrismullins/enable-superbuild-slicer-extension

COMP: Enable superbuild for Slicer4 extension.
BUG: Use generator expression to specify test targets

Fixes tests not running with extension build

Change-Id: Ic049d254c7e4249fc0ce947212e9bd598e7b6626
Merge pull request #167 from chrismullins/specify-test-target-with-generator-expression

BUG: Use generator expression to specify test targets