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
Merge topic 'FixWIN32Missing' into next

928b8fa BUG: Use _WIN32 for detecting windows