Support group options where 1 of n values will be selected (or a default
value will be used). After processing an OPT_FOO will be set to one
value from FOO_OPTIONS for each FOO in SINGLE_OPTIONS. If the user
sets FOO that value will be used, otherwise FOO_DEFAULT will be used
(if FOO_DEFAULT is not explicitly defined it will be set to the first
value in FOO_OPTIONS).
This is somewhat inspired by OPTIONS_SINGLE from ports, but the
structure is quite different with a per-option variable a la MK_FOO.