Fix build using Windows SDK 10.0.17763

c:\qt\qt5\qtbase\src\3rdparty\libjpeg\src\jmorecfg.h(242): error C2371: 'boolean': redefinition; different basic types
c:\program files (x86)\windows kits\10\include\10.0.17763.0\shared\rpcndr.h(193): note: see declaration of 'boolean'

Instead of trying to guess if a certain header has been #included and
whether that header does typedef something to boolean, let's just use
the preprocessor to hide, like some people do for X11/Xlib.h's Bool (see
qmetatype.h where we refused).

Change-Id: I05b8d7ba19004af99f3cfffd15693a87e175f05d
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/plugins/imageformats/jpeg/qjpeghandler.cpp

@@ -56,13 +56,12 @@
 
 // including jpeglib.h seems to be a little messy
 extern "C" {
-// mingw includes rpcndr.h but does not define boolean
-#if defined(Q_OS_WIN) && defined(Q_CC_GNU)
-#   if defined(__RPCNDR_H__) && !defined(boolean)
-        typedef unsigned char boolean;
-#       define HAVE_BOOLEAN
-#   endif
+// jpeglib.h->jmorecfg.h tries to typedef int boolean; but this conflicts with
+// some Windows headers that may or may not have been included
+#ifdef HAVE_BOOLEAN
+#  undef HAVE_BOOLEAN
 #endif
+#define boolean jboolean
 
 #define XMD_H           // shut JPEGlib up
 #include <jpeglib.h>