Fixes building scope outside of a container.

[foot]

Fixes #900

[peterbourgon]

I don't think an export in a Makefile is a good idea. Typically you'll do something like

GO ?= env GO15VENDOREXPERIMENT=1 go

target:
    $(GO) build

but I defer to either @paulbellamy or @tomwilkie on this.

[foot]

Yes! I will update this PR w/ Peter's suggestion!

[davkal]

Running this, still wants to build the docker container:

$ make SUDO= BUILD_IN_CONTAINER=0
time go build -i -ldflags "-extldflags \"-static\" -X main.version=abba6cf" -tags netgo -o prog/scope ./prog

real    0m44.775s
user    1m19.340s
sys 0m8.134s
cp prog/scope vendor/runsvinit/runsvinit docker/
cp .pkg/docker-1.6.2.tgz docker/docker.tgz
docker build -t weaveworks/scope docker/
Post http://%2Fvar%2Frun%2Fdocker.sock/v1.20/build?cgroupparent=&cpuperiod=0&cpuquota=0&cpusetcpus=&cpusetmems=&cpushares=0&dockerfile=Dockerfile&memory=0&memswap=0&rm=1&t=weaveworks%2Fscope&ulimits=null: dial unix /var/run/docker.sock: connect: no such file or directory.
* Are you trying to connect to a TLS-enabled daemon without TLS?
* Is your docker daemon up and running?
make: *** [scope.tar] Error 1

On the other hand, if I just run the following in master (without GO15VENDOREXPERIMENT) everything is fine:

$ make BUILD_IN_CONTAINER=false prog/scope
time go build -i -ldflags "-extldflags \"-static\" -X main.version=1b554ba" -tags netgo -o prog/scope ./prog

real    0m14.122s
user    0m14.997s
sys 0m2.795s

Is this PR still needed?

[foot]

That is weird! Doesn't work for me on master =/

I will update this PR though.

ewq:~/weave/src/github.com/weaveworks/scope(master $%=)$ make BUILD_IN_CONTAINER=false prog/scope
time go build -i -ldflags "-extldflags \"-static\" -X main.version=05d4ce0" -tags netgo -o prog/scope ./prog
# k8s.io/kubernetes/pkg/util/parsers
../../../k8s.io/kubernetes/pkg/util/parsers/parsers.go:30: undefined: parsers.ParseRepositoryTag
# github.com/weaveworks/scope/probe/docker
probe/docker/registry.go:69: undefined: docker.CloseWaiter
probe/docker/registry.go:71: undefined: docker.CloseWaiter
probe/docker/registry.go:75: cannot use *docker.Client as type Client in return argument:
        *docker.Client does not implement Client (missing AttachToContainerNonBlocking method)

real    0m1.927s
user    0m0.825s
sys     0m0.585s
make: *** [prog/scope] Error 2

[foot]

@peterbourgon can you have another quick look when you get a sec. Ta!

[peterbourgon]

So all this does is enforce GO15VENDOREXPERIMENT? — If yes, LGTM once Circle is green.