You shouldn't need to undefine IP_UNICAST_IF, since it's not in bits/in.h it shouldn't be defined at all (linux/in.h is not included, as it has conflicts with bits/in.h). Alternatively, you can use configure to define the flag for you, e.g.:
CFLAGS="-DIP_UNICAST_IF=50" ./configure
You shouldn't need to undefine IP_UNICAST_IF, since it's not in bits/in.h it shouldn't be defined at all (linux/in.h is not included, as it has conflicts with bits/in.h). Alternatively, you can use configure to define the flag for you, e.g.: "-DIP_UNICAST_ IF=50" ./configure
CFLAGS=