define public symbols without build constraints

doc.go

@@ -2,9 +2,14 @@
 package pty
 
 import (
+	"errors"
 	"os"
 )
 
+// ErrUnsupported is returned if a function is not
+// available on the current platform.
+var ErrUnsupported = errors.New("unsupported")
+
 // Opens a pty and its corresponding tty.
 func Open() (pty, tty *os.File, err error) {
 	return open()

pty_unsupported.go

@@ -3,14 +3,9 @@
 package pty
 
 import (
-	"errors"
 	"os"
 )
 
-var (
-	ErrUnsupported = errors.New("Unsupported")
-)
-
 func open() (pty, tty *os.File, err error) {
 	return nil, nil, ErrUnsupported
 }