configure.py: don't allow to run multiple 'ninja -C seastar' on same time
configure.py: don't allow to run multiple 'ninja -C seastar' on same time

Scylla's build.ninja allows to run multiple 'ninja -C seastar' on same time,
it breaks DPDK build after upgraded to DPDK-16.10:
https://gist.github.com/syuu1228/4bd1170630b7e5f15653281b4728e521

To prevent it, we need to limit number of seastar build only one in same time.

Note: it doesn't mean disabling parallel build on Seastar.

Signed-off-by: Takuya ASADA <4ebfb6f510af412d6e89fefa3ab96d9c8b5e3fff@scylladb.com>
Message-Id: <1482250560-20289-1-git-send-email-4ebfb6f510af412d6e89fefa3ab96d9c8b5e3fff@scylladb.com>

scylladb/scylla,scylladb/scylla,avikivity/scylla,raphaelsc/scylla,raphaelsc/scylla,avikivity/scylla,duarten/scylla,raphaelsc/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,scylladb/scylla