Fix typo in ITK_WRAPPING variable.
Allow the use of a system ITK build with a SuperBuild.

This allows the use of the superbuild swig with a system ITK.
Merge topic 'SIMPLEITK-366_SystemITKWithSuperbuild'

2589e0b Allow the use of a system ITK build with a SuperBuild.
784a4b3 Fix typo in ITK_WRAPPING variable.