okay, seems to be related to https://www.mongodb.com/community/forums/t/mongo-cxx-driver-4-0-0-error-when-building-from-source-on-linux-mint/307094
If the build folder gets deleted from the tarball (which i was doing to clean the build between attempts), it will fail with the above error. Someone should probably modify the tarball and setup process so that this doesnt happen… since in basically every other situation, the build folder is entirely made up of generated files, and is therefore safe to delete for cleaning