Don't unconditionally delete the zfp build dir
Don't unconditionally delete the zfp build dir

Try to avoid unnecessary recompiles on subsequent runs of
install_deps.sh

mockingbirdnest/Principia,pleroy/Principia,pleroy/Principia,mockingbirdnest/Principia,pleroy/Principia,mockingbirdnest/Principia,pleroy/Principia,mockingbirdnest/Principia