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/cropwindow.cc

@@ -17,7 +17,7 @@
  *  along with RawTherapee.  If not, see <https://www.gnu.org/licenses/>.
  */
 #include <iomanip>
-#ifdef WIN32
+#ifdef _WIN32
 #include <windows.h>
 #endif
 
@@ -1496,7 +1496,7 @@ void CropWindow::expose (Cairo::RefPtr<Cairo::Context> cr)
 
             // While the Right-side ALT is pressed, auto-enable highlight and shadow clipping indicators
             // TODO: Add linux/MacOS specific functions for alternative
-#ifdef WIN32
+#ifdef _WIN32
 
             if (GetKeyState(VK_RMENU) < 0) {
                 showcs = true;