added win32 executable option

Source/cmTarget.h

@@ -54,7 +54,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 class cmTarget
 {
 public:
-  enum TargetType { EXECUTABLE, LIBRARY, UTILITY };
+  enum TargetType { EXECUTABLE, WIN32_EXECUTABLE, LIBRARY, UTILITY };
   /**
    * Return the type of target.
    */