make install only works once

`make install` shouldn't fail if a directory it created still exists.
In this case we can blow away the spl src directory before recreating
it.  This also gracefully handles the migration from pre-spl-merge to
post-spl-merge.

Reviewed by: Richard Elling <[email protected]>
Reviewed-by: Pavel Zakharov <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: George Melikov <[email protected]>
Signed-off-by: Matthew Ahrens <[email protected]>
Closes #7580 

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