textproc/sablotron: Unbreak with Clang 6

Configure fails with

In file included from conftest.cc:29:
In file included from /usr/local/include/expat.h:45:
In file included from /usr/include/c++/v1/stdlib.h:94:
/usr/include/stdlib.h:97:1: error: function declared '[[noreturn]]' after its first declaration
_Noreturn void   exit(int);
^
/usr/include/sys/cdefs.h:280:22: note: expanded from macro '_Noreturn'
#define _Noreturn               [[noreturn]]
                                  ^
conftest.cc:20:6: note: declaration missing '[[noreturn]]' attribute is here
void exit (int);
     ^

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p474051_s336054/logs/Sablot-1.0.3_4.log

textproc/sablotron/Makefile

@@ -17,6 +17,7 @@ LIB_DEPENDS=	libexpat.so:textproc/expat2
 
 GNU_CONFIGURE=	yes
 USES=		gmake iconv libtool localbase:ldflags
+USE_CXXSTD=	c++98
 USE_LDCONFIG=	yes
 LDFLAGS+=	${ICONV_LIB}