cgo can be a trouble

This week, I have to upgrade nff-go from v0.7.0 to v0.8.1, so I change the version first. However, I found the whole package nff-go/low move to nff-go/internal/low, and our code directly based on low. After some research, I found all I need is call C function directly; the function is under librte_ethdev. So I add

// #include <rte_ethdev.h>
import "C"

into our code. I thought since nff-go already link DPDK, I have nothing to do here, but I'm wrong. The problem is that CGO would link C objects into temporary objects first. So linker would complain there was no reference to the function I call. So I add another line:

// #cgo LDFLAGS: -lrte_distributor -lrte_reorder -lrte_kni -lrte_pipeline -lrte_table -lrte_port -lrte_timer -lrte_jobstats -lrte_lpm -lrte_power -lrte_acl -lrte_meter -lrte_sched -lrte_vhost -lrte_ip_frag -lrte_cfgfile -Wl,--whole-archive -Wl,--start-group -lrte_kvargs -lrte_mbuf -lrte_hash -lrte_ethdev -lrte_mempool -lrte_ring -lrte_mempool_ring -lrte_eal -lrte_cmdline -lrte_net -lrte_bus_pci -lrte_pci -lrte_bus_vdev -lrte_timer -lrte_pmd_bond -lrte_pmd_vmxnet3_uio -lrte_pmd_virtio -lrte_pmd_cxgbe -lrte_pmd_enic -lrte_pmd_i40e -lrte_pmd_fm10k -lrte_pmd_ixgbe -lrte_pmd_e1000 -lrte_pmd_ena -lrte_pmd_ring -lrte_pmd_af_packet -lrte_pmd_null -Wl,--end-group -Wl,--no-whole-archive -lrt -lm -ldl -lnuma

They are directly copied from https://github.com/intel-go/nff-go/blob/v0.8.1/internal/low/low_no_mlx.go. However, linker unhappy with it, there are multiple definitions of symbols in the final object now, because we link two DPDK now.In pure CGO, we have no way to link only one at this situation.However, since we know the duplicate references are the same one so whatever the linker picks the program would work. Once we know that, we can use #cgo LDFLAGS: -Wl,--allow-multiple-definition to force the linker to ignore this duplicate. However, we won't want to have a copied from nff-go, so I did the trick in Makefile to copied it automatically when building.