commit 85cd72c0a80d64ddbfd3156743037925bb7f8f5f
"Add the drd/tests/bug322621 regression test"
introduced make distclean failure.
Fix it by using symlink instead of a relative path in
drd/tests/Makefile.
Also, revert "ignore .dirstamp file appearing in helgrind/tests directory"
This reverts commit 718b47e184bc090b9f2a5f22904f59a272c9018d.