-ldflags 'flag list'
 arguments to pass on each go tool link invocation.

-X importpath.name=value
 Set the value of the string variable in importpath named name to value.
 Note that before Go 1.5 this option took two separate arguments.
 Now it takes one argument split on the first = sign.

package main

import ( 
 "fmt"
)

var _VERSION_ = "unknown"

func main() { 
 fmt.Printf("Version:[%s]\n", _VERSION_)
}

export TAG=dev-xxxx 
go build -ldflags "-X main._VERSION_='$TAG'"