$GOROOT/src/sample/app/main.go
$GOROOT/src/sample/app/aflag.go
$GOROOT/src/sample/app/zflag.go

package main

import "fmt"

type appConfig struct {
    Debug bool
    Mode string
}

var config *appConfig

func init() {
    fmt.Println("main::init()");
    if config == nil {
        fmt.Println("main::set_config()");
        config = &appConfig{
            Debug: true,
            Mode: "Development",
        }
    }
}
func main() {
    fmt.Println("Current config is:", config)
}

// +build aflag

package main

import "fmt"

func init() {
    fmt.Println("aflag::init()");
    if config == nil {
        fmt.Println("aflag::set_config()");
        config = &appConfig{
            Debug: false,
            Mode: "Production",
        }
    }
}

// +build zflag

package main

import "fmt"

func init() {
    fmt.Println("zflag::init()");
    if config == nil {
        fmt.Println("zflag::set_config()");
        config = &appConfig{
            Debug: false,
            Mode: "Testing",
        }
    }
}

$ go build             -o bin/main   sample/app
$ go build -tags aflag -o bin/main-a sample/app
$ go build -tags zflag -o bin/main-z sample/app

$ ./bin/main
main::init()
main::set_config()
Current config is: &{true Development}

$ /bin/main-a
aflag::init()
aflag::set_config()
main::init()
Current config is: &{false Production}

$ /bin/main-z
main::init()
main::set_config()
zflag::init()
Current config is: &{true Development}