glr.c: prefer true/false to 1/0 in C++

* data/glr.c: here.

data/glr.c

@@ -293,11 +293,13 @@ b4_percent_code_get[]dnl
 
 #ifdef __cplusplus
   typedef bool yybool;
+# define yytrue true
+# define yyfalse false
 #else
   typedef unsigned char yybool;
+# define yytrue 1
+# define yyfalse 0
 #endif
-#define yytrue 1
-#define yyfalse 0
 
 #ifndef YYSETJMP
 # include <setjmp.h>