Add netgo guard to the Makefile

[tomwilkie]

Its still building a dynamically linked binary

# docker run -ti --entrypoint /bin/sh weaveworks/scope
/home/weave # ldd app
        /lib64/ld-linux-x86-64.so.2 (0x7ff40faba000)
        libpthread.so.0 => /lib64/ld-linux-x86-64.so.2 (0x7ff40faba000)
        libc.so.6 => /lib64/ld-linux-x86-64.so.2 (0x7ff40faba000)

[tomwilkie]

Output from offending build:

https://gist.github.com/peterbourgon/7d7c7262688144e0f719

[tomwilkie]

Problem is people need to run

go clean -i net
go install -tags netgo std

[tomwilkie]

Weave Makefile has a stanza to catch this:

    @strings $@ | grep cgo_stub\\\.go >/dev/null || { \
        rm $@; \
        echo "\nYour go standard library was built without the 'netgo' build tag."; \
        echo "To fix that, run"; \
        echo "    sudo go clean -i net"; \
        echo "    sudo go install -tags netgo std"; \
        false; \
    }

We should include that.

[peterbourgon]

That fixes it.