sc_prohibit_stddef_without_use works incorrectly

[Alfred M. Szmidt]

The following syntax-check is broken,

   _stddef_syms_re = NULL|offsetof|ptrdiff_t|size_t|wchar_t
   # Prohibit the inclusion of stddef.h without an actual use.
   sc_prohibit_stddef_without_use:
        h='stddef.h'                                                    \
        re='\<($(_stddef_syms_re)) *\('                                 \
          $(_sc_header_without_use)

This tries to match "NULL (", NULL can exist in other situations:

  gettimeofday (&tv, NULL);

which will not be matched by the above, the same goes for ptrdiff_t,
size_t and wchar_t (as variable type specifiers).