The Windows SDK declares BOOL as an int. Objective C defines BOOl as …

…a char.

Those two types clash.  MinGW's implementation of the Windows SDK uses the WINBOOL
type to avoid this clash.  When compiling natively on Windows, we need to manually
define WINBOOL.
MinGW will define _DEF_WINBOOL_ if it has defined WINBOOL so we can use the same trick
here.
See https://github.com/mingw-w64/mingw-w64/blob/master/mingw-w64-headers/include/ntdef.h#L355

Source/win32/WIN32Server.m

@@ -63,6 +63,18 @@
 
 #include <math.h>
 
+// The Windows SDK declares BOOL as an int.  Objective C defines BOOl as a char.
+// Those two types clash.  MinGW's implementation of the Windows SDK uses the WINBOOL
+// type to avoid this clash.  When compiling natively on Windows, we need to manually
+// define WINBOOL.
+// MinGW will define _DEF_WINBOOL_ if it has defined WINBOOL so we can use the same trick
+// here.
+// See https://github.com/mingw-w64/mingw-w64/blob/master/mingw-w64-headers/include/ntdef.h#L355
+#ifndef _DEF_WINBOOL_
+#define _DEF_WINBOOL_
+typedef int WINBOOL;
+#endif
+
 // To update the cursor..
 static BOOL update_cursor = NO;
 static BOOL should_handle_cursor = NO;

Source/win32/w32_windowdisplay.m

@@ -33,6 +33,18 @@
 #include "win32/WIN32Server.h"
 #include "win32/WIN32Geometry.h"
 
+// The Windows SDK declares BOOL as an int.  Objective C defines BOOl as a char.
+// Those two types clash.  MinGW's implementation of the Windows SDK uses the WINBOOL
+// type to avoid this clash.  When compiling natively on Windows, we need to manually
+// define WINBOOL.
+// MinGW will define _DEF_WINBOOL_ if it has defined WINBOOL so we can use the same trick
+// here.
+// See https://github.com/mingw-w64/mingw-w64/blob/master/mingw-w64-headers/include/ntdef.h#L355
+#ifndef _DEF_WINBOOL_
+#define _DEF_WINBOOL_
+typedef int WINBOOL;
+#endif
+
 static void 
 invalidateWindow(WIN32Server *svr, HWND hwnd, RECT rect)
 {

Tools/win32pbs.m

@@ -40,6 +40,18 @@
 #include <sys/file.h>
 #endif
 
+// The Windows SDK declares BOOL as an int.  Objective C defines BOOl as a char.
+// Those two types clash.  MinGW's implementation of the Windows SDK uses the WINBOOL
+// type to avoid this clash.  When compiling natively on Windows, we need to manually
+// define WINBOOL.
+// MinGW will define _DEF_WINBOOL_ if it has defined WINBOOL so we can use the same trick
+// here.
+// See https://github.com/mingw-w64/mingw-w64/blob/master/mingw-w64-headers/include/ntdef.h#L355
+#ifndef _DEF_WINBOOL_
+#define _DEF_WINBOOL_
+typedef int WINBOOL;
+#endif
+
 @interface Win32PbOwner : NSObject
 {
   NSPasteboard	*_pb;