BUG: Use _WIN32 for detecting windows
BUG: Use _WIN32 for detecting windows

WIN32 is defined by some windows headers. These headers are not
explicitly included in sitkShow. The _WIN32 is intrinsically defined
by the compiler and should be preferred.

Change-Id: I4ddc628239646095dc744c1681be7a1290701e82

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