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