Windows build docs & Makefile targets fail to produce a Windows binary

[dzrw]

System info:

Telegraf version: version 1.0.0-beta3-46-g69e4e86
OS: OSX 10.10.5
Go version: 1.5

Steps to reproduce:

1. go get github.com/influxdata/telegraf into a clean $GOPATH.
2. Undefine $GOOS and $GOARCH
3. make windows
4. file telegraf.exe
5. Observe that telegraf.exe is Mach-O 64-bit executable x86_64.

Expected behavior:

I expected to be able to use the README.md documentation combined with the make windows target to produce a Windows binary (because I'd like to run the latest version which contains a fix to support running telegraf as a Windows service).

Actual behavior:

From a clean $GOPATH, running make windows fails during go build with missing packages that are not restored by gdm.

../../aws/aws-sdk-go/aws/credentials/shared_credentials_provider.go:8:2: cannot find package "github.com/go-ini/ini" in any of:
    /usr/local/go/src/github.com/go-ini/ini (from $GOROOT)
    /Users/politician/go/src/github.com/go-ini/ini (from $GOPATH)
../../aws/aws-sdk-go/aws/awsutil/path_value.go:9:2: cannot find package "github.com/jmespath/go-jmespath" in any of:
    /usr/local/go/src/github.com/jmespath/go-jmespath (from $GOROOT)
    /Users/politician/go/src/github.com/jmespath/go-jmespath (from $GOPATH)
../../stretchr/testify/assert/assertions.go:18:2: cannot find package "github.com/pmezard/go-difflib/difflib" in any of:
    /usr/local/go/src/github.com/pmezard/go-difflib/difflib (from $GOROOT)
    /Users/politician/go/src/github.com/pmezard/go-difflib/difflib (from $GOPATH)
../../stretchr/testify/mock/mock.go:12:2: cannot find package "github.com/stretchr/objx" in any of:
    /usr/local/go/src/github.com/stretchr/objx (from $GOROOT)
    /Users/politician/go/src/github.com/stretchr/objx (from $GOPATH)
../../hpcloud/tail/watch/inotify.go:12:2: cannot find package "gopkg.in/fsnotify.v1" in any of:
    /usr/local/go/src/gopkg.in/fsnotify.v1 (from $GOROOT)
    /Users/politician/go/src/gopkg.in/fsnotify.v1 (from $GOPATH)
../../hpcloud/tail/watch/inotify.go:13:2: cannot find package "gopkg.in/tomb.v1" in any of:
    /usr/local/go/src/gopkg.in/tomb.v1 (from $GOROOT)
    /Users/politician/go/src/gopkg.in/tomb.v1 (from $GOPATH)

After manually fetching the HEAD of each of these packages, the build was able to complete.

> go get github.com/go-ini/ini
> go get github.com/jmespath/go-jmespath
> go get github.com/pmezard/go-difflib/difflib
> go get github.com/stretchr/objx
> go get gopkg.in/fsnotify.v1
> go get gopkg.in/tomb.v1

However, the binary produced is not a Windows binary!

> file telegraf.exe
telegraf.exe: Mach-O 64-bit executable x86_64
> ./telegraf.exe --version
Telegraf - version 1.0.0-beta3-46-g69e4e86

[dzrw]

Resolved by #1621