gdal @3.4.0 +poppler: Build Failure

[ashimog]

Poppler 22+ requires c++17 while this port still uses c++14.

/opt/local/include/poppler/Form.h:326:10: error: no template named 'optional' in namespace 'std'
    std::optional<GooString> getCheckedSignature(Goffset *checkedFileSize);
    ~~~~~^
/opt/local/include/poppler/Form.h:619:10: error: no template named 'optional' in namespace 'std'
    std::optional<GooString> getCheckedSignature(Goffset *checkedFileSize);
    ~~~~~^
2 errors generated.

This is the exactly same issue as ​https://github.com/OSGeo/gdal/issues/5071.

Change compiler.cxx_standard to 2017, and
add configure.cxxflags-append -std=c++17 solves this problem.