Correctly include the default 'extra' options or command

If a project defines "extra.options += ...", the default
values of ${vars.base.extra} are included. However, if
the project defines "extra: {...", the default extra
in ${vars.base} is overridden; in that case,the definition
should really be "extra: ${vars.base.extra} {...", in
order to extend correctly the default extra values defined
in ${vars.base}.