packaging: Fixed problem that bin/umount.xtreemfs was deleted by "make clean"

when building the source tarball. (Fix in revision 3655 did not work if the directory cpp/build does not exist.)

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3678 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534