kaldi uses the C++17 feature while compiling in the C++14 mode

[yurivict]

/usr/local/libexec/ccache/c++ -DHAVE_CLAPACK=1 -DKALDI_NO_PORTAUDIO=1 -I/usr/ports/audio/kaldi/work/kaldi-21ae411/tools/CLAPACK -I/usr/ports/audio/kaldi/work/kaldi-21ae411/src/tree/.. -I/usr/ports/audio/kaldi/work/kaldi-21ae411/src/util/.. -I/usr/ports/audio/kaldi/work/kaldi-21ae411/src/matrix/.. -I/usr/ports/audio/kaldi/work/kaldi-21ae411/src/base/.. -O2 -pipe -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing  -isystem /usr/local/include -O2 -pipe -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing  -isystem /usr/local/include  -DNDEBUG -std=c++14 -pthread -MD -MT src/tree/CMakeFiles/kaldi-tree.dir/tree-renderer.cc.o -MF src/tree/CMakeFiles/kaldi-tree.dir/tree-renderer.cc.o.d -o src/tree/CMakeFiles/kaldi-tree.dir/tree-renderer.cc.o -c /usr/ports/audio/kaldi/work/kaldi-21ae411/src/tree/tree-renderer.cc
In file included from /usr/ports/audio/kaldi/work/kaldi-21ae411/src/tree/tree-renderer.cc:20:
In file included from /usr/ports/audio/kaldi/work/kaldi-21ae411/src/tree/../tree/tree-renderer.h:26:
In file included from /usr/local/include/fst/fstlib.h:42:
In file included from /usr/local/include/fst/expanded-fst.h:33:
In file included from /usr/local/include/fst/fst.h:41:
In file included from /usr/local/include/fst/arc.h:29:
In file included from /usr/local/include/fst/error-weight.h:21:
/usr/local/include/fst/util.h:285:6: error: no template named 'optional' in namespace 'std'
std::optional<int64> ParseInt64(std::string_view s);
~~~~~^

clang-16
FreeBSD 13.2