textproc/html2text: fix build with clang 16

Since clang 16 (and gcc 11) the default C++ standard is now gnu++17.
Because textproc/html2text's Makefile does not explicitly set its C++
standard, this leads to several errors:

  /usr/local/lib/bison.cc:429:3: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    register int yystate;
    ^~~~~~~~~
  /usr/local/lib/bison.cc:430:3: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    register int yyn;
    ^~~~~~~~~
  /usr/local/lib/bison.cc:431:3: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    register short *yyssp;
    ^~~~~~~~~
  /usr/local/lib/bison.cc:432:3: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    register YY_HTMLParser_STYPE *yyvsp;
    ^~~~~~~~~

Add USE_CXXSTD=gnu++98 to avoid these errors.

PR:		271031
Approved by:	maintainer timeout (2 weeks)
MFH:		2023Q2

textproc/html2text/Makefile

@@ -1,5 +1,6 @@
 PORTNAME=	html2text
 PORTVERSION=	1.3.2a
+PORTREVISION=	1
 PORTEPOCH=	1
 CATEGORIES=	textproc
 MASTER_SITES=	SUNSITE/apps/www/converters \
@@ -13,6 +14,7 @@ LICENSE=	GPLv2+
 LICENSE_FILE=	${WRKSRC}/COPYING
 
 HAS_CONFIGURE=	yes
+USE_CXXSTD=	gnu++98
 
 PORTDOCS=	CHANGES CREDITS KNOWN_BUGS README TODO
 PLIST_FILES=	bin/html2text man/man1/html2text.1.gz man/man5/html2textrc.5.gz