make install only works once

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 <Richard.Elling@RichardElling.com>
Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #7580