configure: split a ld_flags_{mode} out of cxxflags_{mode}
configure: split a ld_flags_{mode} out of cxxflags_{mode}

Flags that we want to pass to gcc during compilation and linking are
in cxx_ld_flags_{mode}.

With this patch, we no longer pass

-I. -I build/{mode}/gen

to the link, which should have no impact.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla