Fix [DE]SELECTED_OPTIONS to lazily evaluate as late as possible.

If a port modifies OPTIONS_UNSET after including bsd.port.options.mk
then these values were wrong even though all of 'showconfig',
'pretty-print-config', and the package generated all had the expected
options.

Only Poudriere and synth use these variables for incremental build checks.

Reviewed by: sjg, bapt
Approved by: portmgr (bapt)
Differential Revision: https://reviews.freebsd.org/D13092