pam: fix test usage in configure script

The standard test command does not support the == operator. Certain
shells, including bash, do support it, but in those shells it does
exactly the same thing as the standard = operator. Use that instead.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Harald van Dijk <harald@gigawatt.nl>
Closes #10509