Make Q_NULLPTR be NULL on C++98

Some compilers actually have a C++11-like null pointer constant in C++98
mode:

$ gcc -dM -E -include stddef.h -xc++ /dev/null | grep NULL
#define NULL __null
$ icc -dM -E -include stddef.h -xc++ /dev/null | grep NULL
#define NULL __null
$ clang -dM -E -include stddef.h -xc++ /dev/null | grep NULL
#define NULL __null

Change-Id: Ie0bcaf36fed6ad27e761a0a24332817129128571
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/global/qcompilerdetection.h

@@ -858,7 +858,7 @@
 #ifdef Q_COMPILER_NULLPTR
 # define Q_NULLPTR         nullptr
 #else
-# define Q_NULLPTR         0
+# define Q_NULLPTR         NULL
 #endif
 
 #ifdef Q_COMPILER_DEFAULT_MEMBERS