fix-ltdl: Delete libobj symlink before attempting to create it.

fix-ltdl: Delete libobj symlink before attempting to create it.

There seems to be a (rare) race in parallel builds where make for some
reason decides to update the libobj symlink after it already exists,
which fails.  I can't reproduce the failure, but deleting the symlink
(if one exists) before creating it should avoid the issue.