-# b4_define_flag_if(FLAG)
-# -----------------------
-# Define "b4_FLAG_if(IF-TRUE, IF-FALSE)" that depends on the
-# value of the Boolean FLAG.
-m4_define([b4_define_flag_if],
-[_b4_define_flag_if($[1], $[2], [$1])])
-
-# _b4_define_flag_if($1, $2, FLAG)
-# --------------------------------
-# This macro works around the impossibility to define macros
-# inside macros, because issuing `[$1]' is not possible in M4 :(.
-# This sucks hard, GNU M4 should really provide M5 like $$1.
-m4_define([_b4_define_flag_if],
-[m4_if([$1$2], $[1]$[2], [],
- [m4_fatal([$0: Invalid arguments: $@])])dnl
-m4_define([b4_$3_if],
- [b4_flag_if([$3], [$1], [$2])])])
-
-
-# b4_FLAG_if(IF-TRUE, IF-FALSE)
-# -----------------------------
-# Expand IF-TRUE, if FLAG is true, IF-FALSE otherwise.
-b4_define_flag_if([defines]) # Whether headers are requested.
-b4_define_flag_if([error_verbose]) # Whether error are verbose.
-b4_define_flag_if([locations]) # Whether locations are tracked.
-b4_define_flag_if([pure]) # Whether the interface is pure.
-b4_define_flag_if([yacc]) # Whether POSIX Yacc is emulated.
+
+# b4_null_define
+# --------------
+# Portability issues: define a YY_NULL appropriate for the current
+# language (C, C++98, or C++11).
+m4_define([b4_null_define],
+[# ifndef YY_NULL
+# if defined __cplusplus && 201103L <= __cplusplus
+# define YY_NULL nullptr
+# else
+# define YY_NULL 0
+# endif
+# endif[]dnl
+])
+
+
+# b4_null
+# -------
+# Return a null pointer constant.
+m4_define([b4_null], [YY_NULL])