Enable IP_BIND_ADDRESS_NO_PORT on backend connections when the source
address is specified without port or port ranges. This is supported
since Linux 4.2/libc 2.23.
If the kernel supports it but the libc doesn't, we can define it at
build time:
make [...] DEFINE=-DIP_BIND_ADDRESS_NO_PORT=24
For more informations about this feature, see Linux commit 90c337da