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/main.cc

@@ -46,7 +46,7 @@
 #include "../rtengine/procparams.h"
 #include "pathutils.h"
 
-#ifndef WIN32
+#ifndef _WIN32
 #include <glibmm/fileutils.h>
 #include <glib.h>
 #include <glib/gstdio.h>
@@ -121,7 +121,7 @@ int processLineParams ( int argc, char **argv )
                     // GTK --argument, we're skipping it
                     break;
 
-#ifdef WIN32
+#ifdef _WIN32
 
                 case 'w': // This case is handled outside this function
                     break;
@@ -167,7 +167,7 @@ int processLineParams ( int argc, char **argv )
                     printf("  %s <file>             Start Image Editor with file.\n\n",Glib::path_get_basename (argv[0]).c_str());
                     std::cout << std::endl;
                     printf("Options:\n");
-#ifdef WIN32
+#ifdef _WIN32
                     printf("  -w Do not open the Windows console\n");
 #endif
                     printf("  -v Print RawTherapee version number and exit\n");
@@ -210,7 +210,7 @@ bool init_rt()
         TIFFSetWarningHandler (nullptr);   // avoid annoying message boxes
     }
 
-#ifndef WIN32
+#ifndef _WIN32
 
     // Move the old path to the new one if the new does not exist
     if (Glib::file_test (Glib::build_filename (options.rtdir, "cache"), Glib::FILE_TEST_IS_DIR) && !Glib::file_test (options.cacheBaseDir, Glib::FILE_TEST_IS_DIR)) {
@@ -362,7 +362,7 @@ int main (int argc, char **argv)
     Glib::init();  // called by Gtk::Main, but this may be important for thread handling, so we call it ourselves now
     Gio::init ();
 
-#ifdef WIN32
+#ifdef _WIN32
     if (GetFileType (GetStdHandle (STD_OUTPUT_HANDLE)) == 0x0003) {
         // started from msys2 console => do not buffer stdout
         setbuf(stdout, NULL);
@@ -373,7 +373,7 @@ int main (int argc, char **argv)
     char exname[512] = {0};
     Glib::ustring exePath;
     // get the path where the rawtherapee executable is stored
-#ifdef WIN32
+#ifdef _WIN32
     WCHAR exnameU[512] = {0};
     GetModuleFileNameW (NULL, exnameU, 511);
     WideCharToMultiByte (CP_UTF8, 0, exnameU, -1, exname, 511, 0, 0 );
@@ -416,7 +416,7 @@ int main (int argc, char **argv)
     options.rtSettings.lensfunDbBundleDirectory = LENSFUN_DB_PATH;
 #endif
 
-#ifdef WIN32
+#ifdef _WIN32
     bool consoleOpened = false;
 
     // suppression of annoying error boxes
@@ -567,7 +567,7 @@ int main (int argc, char **argv)
         }
     }
 
-#ifdef WIN32
+#ifdef _WIN32
 
     if (consoleOpened) {
         printf ("Press any key to exit RawTherapee\n");