Check GOPATH from 'go env GOPATH' when running 'make'

[jiekang]

When running 'make', the GOPATH environment variable is used to determine if the project is in the correct location. This patch changes this step to also try to determine GOPATH from 'go env GOPATH'.

Addresses #1728

[centos-ci]

Can one of the admins verify this patch?

[centos-ci]

Can one of the admins verify this patch?

[baijum]

[test]

[baijum]

Looks like there are still few more errors:

$ make
vendor/github.com/goadesign/goa/goagen/goagen app -d github.com/fabric8-services/fabric8-wit/design
invalid plugin package import path: cannot find package "github.com/goadesign/goa/goagen/gen_app" in any of:
        /usr/local/go/src/github.com/goadesign/goa/goagen/gen_app (from $GOROOT)
        ($GOPATH not set. For more details see: 'go help gopath')
make: *** [Makefile:240: app/controllers.go] Error 1

[fabric8cd]

@jiekang snapshot fabric8-wit image is available for testing. docker pull docker.io/fabric8/fabric8-wit:SNAPSHOT-PR-1735-2

[baijum]

[test]

[baijum]

LGTM

[jiekang]

The 'go' tools will assume GOPATH is $HOME/go if not set. This feature is in go 1.8+, not 1.7+. For more info see: golang/go#17262

Applications and libraries do not necessarily make the same assumption, or use 'go env' to determine the GOPATH. In this case, goagen uses os.getEnv("GOPATH") extensively to determine the GOPATH. It's not sane to try to set the GOPATH environment variable for a parent process, and I don't think it's a good idea to patch the f8-wit build to set GOPATH to the value of 'go env GOPATH' prior to running goagen or anything else, 1.8 support notwithstanding. The latest commit in this PR does set GOPATH prior to running goagen, mainly to show what needs to be done. I don't like the solution.

For what it's worth, fabric8-wit does not build without go 1.8+ as one of it's dependencies uses features only available in 1.8+. See: #1736

[fabric8cd]

@jiekang snapshot fabric8-wit image is available for testing. docker pull docker.io/fabric8/fabric8-wit:SNAPSHOT-PR-1735-3