The handling of _select dependencies had two issues:

1) extralibs from _select dependencies of a component were not added to
   the list of extralibs for that component.
2) extralibs from dependencies were only added to the extralibs of a
   component if the component was enabled. This led to incorrect results
   if that component was enabled by another component later in the dependency
   resolution process. Instead, always generate the full list of component
   extralibs for use later in the dependency resolution process.

Also remove a leftover unused variable.