mongodb4 not building under netbsd 9 amd64.

[Greg Troxel <gdt%lexort.com@localhost>]

mongodb4, needed for unifi, fails to build under netbsd-9 amd64 on
2024Q2.

The issue seems to be

/usr/bin/ld: build/59f4f0dd/mongo/shell/dbshell.o: in function `edit(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
dbshell.cpp:(.text+0x447d): undefined reference to `std::__cxx11::basic_stringstream<char, std::char_traits<char>, std::allocator<char> >::basic_stringstream()'
/usr/bin/ld: dbshell.cpp:(.text+0x45ba): undefined reference to `std::__cxx11::basic_stringstream<char, std::char_traits<char>, std::allocator<char> >::basic_stringstream()'
/usr/bin/ld: build/59f4f0dd/mongo/shell/dbshell.o: in function `_main(int, char**)':
dbshell.cpp:(.text+0x609f): undefined reference to `std::__cxx11::basic_stringstream<char, std::char_traits<char>, std::allocator<char> >::basic_stringstream()'


The build is using pkgsrc-built gcc10, which has been fine to build
other things including fortran.

The pkgsrc Makefile has

  USE_CXX_FEATURES=       c++17 filesystem
  GCC_REQD+=              10

So perhaps this needs FORCE_CXX_STD and upstream is not passing it?  Or
perhaps it is missing this on a ld call?