$NetBSD$

Use c++20 to fix compilation errors.

--- jfbuild/Makefile.orig	2023-03-21 20:06:35.000000000 +0000
+++ jfbuild/Makefile
@@ -39,7 +39,7 @@ HOSTCC?=$(CC)
 HOSTCXX?=$(CXX)
 
 OURCFLAGS=-g -W -Wall -fno-strict-aliasing -std=c99
-OURCXXFLAGS=-g -W -Wall -fno-exceptions -fno-rtti -std=c++98
+OURCXXFLAGS=-g -W -Wall -fno-exceptions -fno-rtti -std=c++20
 OURCPPFLAGS=-I$(INC) -I$(SRC) -I$(LIBSQUISH)
 OURLDFLAGS=