restore type of NULL to void * except when used in C++ programs

unfortunately this eliminates the ability of the compiler to diagnose
some dangerous/incorrect usage, but POSIX requires (as an extension to
the C language, i.e. CX shaded) that NULL have type void *. plain C
allows it to be defined as any null pointer constant.

the definition 0L is preserved for C++ rather than reverting to plain
0 to avoid dangerous behavior in non-conforming programs which use
NULL as a variadic sentinel. (it's impossible to use (void *)0 for C++
since C++ lacks the proper implicit pointer conversions, and other
popular alternatives like the GCC __null extension seem non-conforming
to the standard's requirements.)

include/stddef.h

@@ -1,7 +1,11 @@
 #ifndef _STDDEF_H
 #define _STDDEF_H
 
+#ifdef __cplusplus
 #define NULL 0L
+#else
+#define NULL ((void*)0)
+#endif
 
 #define __NEED_ptrdiff_t
 #define __NEED_size_t