[Udpcast] Darwin build fixes patch

[Alain Knaff]

Devin Bayer wrote:
> Hello.  Thanks for developing udpcast.  It almost compiles on my mac.  
> But I have to make a few small changes.
> 
> 1.) the linker doesn't support the -Wl,-warn-common option
> 2.) what is malloc.h?  It isn't needed.
> 3.) struct ifreq is variable size, so we must use _SIZEOF_ADDR_IFREQ(), 
> not pointer arithemetic
> 4.) some endian issues
> 
> Thanks,
> Devin Bayer

This patches are now in today's version (20070129)

Regards,

Alain