remove --prefix when configuring unuran. It is not needed. Thsi should fix a...

remove --prefix when configuring unuran. It is not needed. Thsi should fix a warning appearing in the nightly system due to wrong prefix string


git-svn-id: http://root.cern.ch/svn/root/trunk@26595 27541ba8-7e3a-0410-8455-c3a389f83636