make the install-{examples,ext,tests} targets idempotent
make the install-{examples,ext,tests} targets idempotent

mozilla/disco,pombredanne/disco,scrapinghub/disco,pavlobaron/disco_playground,seabirdzh/disco,pooya/disco,discoproject/disco,beni55/disco,seabirdzh/disco,mozilla/disco,oldmantaiter/disco,discoproject/disco,pombredanne/disco,pombredanne/disco,oldmantaiter/disco,ktkt2009/disco,pooya/disco,ErikDubbelboer/disco,mwilliams3/disco,mwilliams3/disco,pavlobaron/disco_playground,seabirdzh/disco,simudream/disco,beni55/disco,scrapinghub/disco,ErikDubbelboer/disco,simudream/disco,discoproject/disco,scrapinghub/disco,mwilliams3/disco,oldmantaiter/disco,scrapinghub/disco,ErikDubbelboer/disco,mwilliams3/disco,pavlobaron/disco_playground,ErikDubbelboer/disco,simudream/disco,seabirdzh/disco,mozilla/disco,simudream/disco,beni55/disco,simudream/disco,pooya/disco,ktkt2009/disco,ktkt2009/disco,pavlobaron/disco_playground,ErikDubbelboer/disco,mozilla/disco,pooya/disco,ktkt2009/disco,oldmantaiter/disco,mwilliams3/disco,ktkt2009/disco,seabirdzh/disco,pombredanne/disco,pombredanne/disco,beni55/disco,beni55/disco,oldmantaiter/disco,discoproject/disco,discoproject/disco