maint: propagate DEPENDENCIES to libs in single binary mode

maint: propagate DEPENDENCIES to libs in single binary mode

build-aux/gen-single-binary.sh (override_single): A new function
to refactor the existing mappings for dir, vdir, and arch.
This function now also sets the DEPENDENCIES variable so that these
dependencies can be maintained later in the script, where
we now propagate the automake generated $(src_$cmd_DEPENDENCIES)
to our equivalent src_libsinglebin_$cmd_a_DEPENDENCIES.
This will ensure that any required libs are built,
which we require in a following change to cksum that
builds part of it as a separate library.