some libc implementations don't define struct ip_mreqn
define it here if CFLAG passed with -DNEED_STRUCT_IP_MREQN

I found this issue cross compiling for ARM using eclibc in the
https://www.yoctoproject.org/  build system.
Merge pull request #45 from karlhiramoto/master

fix cross-compile:   some libc implementations don't define struct ip_mreqn