Use _WIN32 instead of WIN32 to detect Windows

`WIN32` is not defined when building a 64-bit executable on Windows with Clang. `_WIN32` is the more appropriate option here.
http://web.archive.org/web/20191012035921/http://nadeausoftware.com/articles/2012/01/c_c_tip_how_use_compiler_predefined_macros_detect_operating_system contains a handy table/matrix of the options and why this is best.

rtgui/thumbnail.cc

@@ -15,7 +15,7 @@
  *  You should have received a copy of the GNU General Public License
  *  along with RawTherapee.  If not, see <https://www.gnu.org/licenses/>.
  */
-#ifdef WIN32
+#ifdef _WIN32
 #include <windows.h>
 #endif
 
@@ -1142,7 +1142,7 @@ void Thumbnail::removeThumbnailListener (ThumbnailListener* tnl)
 bool Thumbnail::openDefaultViewer(int destination)
 {
 
-#ifdef WIN32
+#ifdef _WIN32
     Glib::ustring openFName;
 
     if (destination == 1) {