More precisely:
- for "older" libspec in LIB_DEPENDS, only compare them against
the libs from the LIB_DEPENDS.
- check LIB_DEPENDS against all libspecs for the package (both from WANTLIB
and LIB_DEPENDS) to turn them into LIB_DEPENDS.
- create a full list of installed libs and of 'inter-package' libraries to
solve all WANTLIB in one go.
This is actually faster than the old code, but a bit more complicated.