mail/filtermail fails to build with 13.0-CURRENT

[Mike Clarke]

As someone with very little experience with C++ I'd welcome advice on how to fix this for 13.0-
CURRENT. It builds without problem on 12.1-RELEASE

Making all in src
gmake[3]: Entering directory '/usr/ports/mail/filtermail/work/filtermail-0.8.3/src'
c++ -DHAVE_CONFIG_H -I. -I.. -I/usr/local/include -I. -I../include -I.. -DLOCALEDIR=\"/usr/local/share/
locale\" -I../intl -I.. -I../include -I.  -I/usr/local/include -Wall -O2 -pipe -fstack-protector-strong -fno-
strict-aliasing   -MT rcfile.o -MD -MP -MF .deps/rcfile.Tpo -c -o rcfile.o rcfile.cc
rcfile.ll:151:14: error: no viable overloaded '='
        yyin = new ifstream (sub_file.c_str ());
        ~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/v1/istream:200:20: note: candidate function not viable: no known conversion from 
'std::__1::ifstream *' (aka 'basic_ifstream<char> *') to 'std::__1::basic_istream<char>' for 1st 
argument; dereference the argument with *
    basic_istream& operator=(basic_istream&& __rhs);
                   ^
/usr/include/c++/v1/istream:211:20: note: candidate function not viable: no known conversion from 
'std::__1::ifstream *' (aka 'basic_ifstream<char> *') to 'const std::__1::basic_istream<char>' for 1st 
argument; dereference the argument with *
    basic_istream& operator=(const basic_istream& __rhs) = delete;
                   ^
rcfile.ll:153:15: error: cannot cast from type 'std::istream' (aka 'basic_istream<char>') to pointer 
type 'std::__1::ifstream *' (aka 'basic_ifstream<char> *')
        if (!((ifstream*) yyin)->is_open ())
              ^~~~~~~~~~~~~~~~
2 errors generated.
gmake[3]: *** [Makefile:453: rcfile.o] Error 1
gmake[3]: Leaving directory '/usr/ports/mail/filtermail/work/filtermail-0.8.3/src'
gmake[2]: *** [Makefile:382: all-recursive] Error 1
gmake[2]: Leaving directory '/usr/ports/mail/filtermail/work/filtermail-0.8.3'
gmake[1]: *** [Makefile:320: all] Error 2
gmake[1]: Leaving directory '/usr/ports/mail/filtermail/work/filtermail-0.8.3'
*** Error code 1
-- 
Mike Clarke