What version of xextproto have you used? Because in Xext.h provided by xextproto-7.0.3 there is:
Code:
extern int (*XSetExtensionErrorHandler(
int (*handler)(
Display *,
char *,
char *
)
))(
Display *,
char *,
char *
);
Note also that extutil.c is in libXext-1.0.4/src/extutil.c