check to see if tweetnacl has been built
check to see if tweetnacl has been built

if it has, avoid rebuilding

dmpiergiacomo/scion,dmpiergiacomo/scion,FR4NK-W/osourced-scion,klausman/scion,Oncilla/scion,dmpiergiacomo/scion,FR4NK-W/osourced-scion,klausman/scion,klausman/scion,Oncilla/scion,netsec-ethz/scion,FR4NK-W/osourced-scion,FR4NK-W/osourced-scion,FR4NK-W/osourced-scion,netsec-ethz/scion,Oncilla/scion,dmpiergiacomo/scion,klausman/scion,dmpiergiacomo/scion,klausman/scion,Oncilla/scion,dmpiergiacomo/scion,netsec-ethz/scion,netsec-ethz/scion