Remove old external build directory before creating new one.
Remove old external build directory before creating new one.

Otherwise, remainents of failed externals builds can cause new builds
to fail as well.

zhangzhehust/htcondor,neurodebian/htcondor,clalancette/condor-dcloud,clalancette/condor-dcloud,htcondor/htcondor,neurodebian/htcondor,htcondor/htcondor,bbockelm/condor-network-accounting,djw8605/condor,zhangzhehust/htcondor,djw8605/htcondor,djw8605/condor,djw8605/condor,djw8605/htcondor,clalancette/condor-dcloud,djw8605/condor,bbockelm/condor-network-accounting,djw8605/htcondor,mambelli/osg-bosco-marco,htcondor/htcondor,mambelli/osg-bosco-marco,djw8605/condor,zhangzhehust/htcondor,neurodebian/htcondor,djw8605/condor,djw8605/htcondor,djw8605/htcondor,htcondor/htcondor,mambelli/osg-bosco-marco,htcondor/htcondor,mambelli/osg-bosco-marco,clalancette/condor-dcloud,clalancette/condor-dcloud,djw8605/condor,zhangzhehust/htcondor,neurodebian/htcondor,djw8605/htcondor,djw8605/condor,bbockelm/condor-network-accounting,mambelli/osg-bosco-marco,djw8605/htcondor,bbockelm/condor-network-accounting,neurodebian/htcondor,bbockelm/condor-network-accounting,djw8605/htcondor,mambelli/osg-bosco-marco,htcondor/htcondor,mambelli/osg-bosco-marco,bbockelm/condor-network-accounting,zhangzhehust/htcondor,bbockelm/condor-network-accounting,djw8605/htcondor,neurodebian/htcondor,bbockelm/condor-network-accounting,mambelli/osg-bosco-marco,neurodebian/htcondor,neurodebian/htcondor,zhangzhehust/htcondor,zhangzhehust/htcondor,htcondor/htcondor,clalancette/condor-dcloud,neurodebian/htcondor,zhangzhehust/htcondor,clalancette/condor-dcloud,htcondor/htcondor,zhangzhehust/htcondor