sunos: allow suppression of ifaddrs.h
sunos: allow suppression of ifaddrs.h

Older Solaris versions do not have this feature.  This allows
them to set SUNOS_NO_IFADDRS to allow libuv to build.

Re: joyent/node#3465

urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2