One-line fix in buildsystem only targetting bug lp:920446.
One-line fix in buildsystem only targetting bug lp:920446.

This checks for an empty DESTDIR and replaces such with / rather than supplying
an empty --root= which was causing problems with distutils.

Thanks to Patrick, Florian, and Stephan for input.

jjo31/ATHAM-Fluidity,jrper/fluidity,rjferrier/fluidity,jrper/fluidity,rjferrier/fluidity,iakovos-panourgias/fluidity,jjo31/ATHAM-Fluidity,jjo31/ATHAM-Fluidity,jjo31/ATHAM-Fluidity,rjferrier/fluidity,rjferrier/fluidity,iakovos-panourgias/fluidity,iakovos-panourgias/fluidity,jrper/fluidity,jrper/fluidity,rjferrier/fluidity,iakovos-panourgias/fluidity,jjo31/ATHAM-Fluidity,jrper/fluidity,iakovos-panourgias/fluidity