Fix packaging on Windows

And also improve to use the "build gui" flag
Fixes #586

CHANGELOG.md

@@ -15,6 +15,7 @@ More detailed release notes can be found on the [releases page](https://github.c
 * Issues when settings changes may not be monitored (#576)
 * Layout of hidden tab container contents on mobile (#578)
 * Mobile apps would not quit when Quit() was called (#580)
+* Fyne package could fail on Windows (#586)
 
 
 ## 1.2 - 12 December 2019

cmd/fyne/build.go

@@ -17,10 +17,15 @@ func (b *builder) build() error {
 		goos = runtime.GOOS
 	}
 
-	cmd := exec.Command("go", "build", b.srcdir)
+	var cmd *exec.Cmd
+	if goos == "windows" {
+		cmd = exec.Command("go", "build", "-ldflags", "-H=windowsgui", ".")
+	} else {
+		cmd = exec.Command("go", "build", ".")
+	}
 	cmd.Dir = b.srcdir
 	env := os.Environ()
-	env = append(env, "GOOS=", goos)
+	env = append(env, "GOOS="+goos)
 	cmd.Env = env
 
 	out, err := cmd.CombinedOutput()