Restore :: in Makefile.am

The double-colon looked like a typo, but it's actually an obscure
feature. Rules with :: may appear multiple times and are run
independently of one another in the order they appear. The use of ::
for distclean-local was conventional, not accidental.

Add comments to indicate the intentional use of double-colon rules.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9210