shells/yash: Add mirror and use framework helper

Add GitHub mirror as backup
Use USE_CSTD helper instead of CFLAGS variable

Reference:
https://yash.osdn.jp/index.html.en

PR:		270724
Reviewed by:	naddy (maintainer)

shells/yash/Makefile

@@ -1,7 +1,8 @@
 PORTNAME=	yash
 DISTVERSION=	2.54
 CATEGORIES=	shells
-MASTER_SITES=	OSDN/yash
+MASTER_SITES=	OSDN/yash \
+		https://github.com/magicant/${PORTNAME}/releases/download/${DISTVERSION}/
 
 MAINTAINER=	naddy@FreeBSD.org
 COMMENT=	POSIX-compliant command line shell
@@ -11,10 +12,10 @@ LICENSE=	GPLv2+
 LICENSE_FILE=	${WRKSRC}/COPYING
 
 USES=		tar:xz
+USE_CSTD=	c99
 
 HAS_CONFIGURE=	yes
 CONFIGURE_ARGS=	--prefix=${PREFIX} --mandir=${MANPREFIX}/man
-CPPFLAGS+=	-std=c99
 
 OPTIONS_DEFINE=	NLS
 OPTIONS_SUB=	yes