Incorporating changes from Slicer and improve style
Incorporating changes from Slicer and improve style

Adding changes in Slicer's version of the file which have a include
guard and a sanity check for Swig_DIR.

Additionally renamed version variable to be prefixed with SWIG.

Added variable with define the SWIG version, and the hash in one place
to aid in all getting updated when a version changes.

Change-Id: I5bc26b2ec425f6e21e9822df08d28342352f6252

blowekamp/SimpleITK,SimpleITK/SimpleITK,kaspermarstal/SimpleElastix,hendradarwin/SimpleITK,InsightSoftwareConsortium/SimpleITK,SimpleITK/SimpleITK,kaspermarstal/SimpleElastix,hendradarwin/SimpleITK,InsightSoftwareConsortium/SimpleITK,blowekamp/SimpleITK,InsightSoftwareConsortium/SimpleITK,richardbeare/SimpleITK,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,blowekamp/SimpleITK,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,richardbeare/SimpleITK,InsightSoftwareConsortium/SimpleITK,hendradarwin/SimpleITK,hendradarwin/SimpleITK,kaspermarstal/SimpleElastix,InsightSoftwareConsortium/SimpleITK,richardbeare/SimpleITK,richardbeare/SimpleITK,blowekamp/SimpleITK,SimpleITK/SimpleITK,richardbeare/SimpleITK,InsightSoftwareConsortium/SimpleITK,InsightSoftwareConsortium/SimpleITK,SimpleITK/SimpleITK,blowekamp/SimpleITK,richardbeare/SimpleITK,SimpleITK/SimpleITK,blowekamp/SimpleITK,InsightSoftwareConsortium/SimpleITK,blowekamp/SimpleITK,hendradarwin/SimpleITK,SimpleITK/SimpleITK,richardbeare/SimpleITK,richardbeare/SimpleITK,blowekamp/SimpleITK,hendradarwin/SimpleITK,hendradarwin/SimpleITK,hendradarwin/SimpleITK,SimpleITK/SimpleITK,SimpleITK/SimpleITK