adding LDFLAGS to linux build instructions

I was recently trying to build noit inside a docker container:
https://github.com/virgo-agent-toolkit/agent-poller-noit-parity (Ubuntu 14.04 image)

I also tried this on an actual Ubuntu cloud server and I still got issues while following the instructions  for building.

console o/p without this change:
https://gist.github.com/ynachiket/2d6ddbd39f0e57362662

console o/p with this change:
https://gist.github.com/ynachiket/bbec6517cd9e1be00b92