when building glibc with libpthread as an addon I get this

configure: running configure fragment for add-on libpthread
configure: WARNING: you should use --build, --host, --target
configure: WARNING: you should use --build, --host, --target
checking build system type... Invalid configuration `dummy': machine
`dummy' not recognized
configure: error:
/gnu/store/281n6ma5jxy9sb3nwc09mzpqx43vczr6-bash-4.2/bin/bash
../source/scripts/config.sub dummy failed

Normally the flags passed to glibc's configure should be passed to
libpthread as well. Why isn't it happening?