cmd: write warning when preserve ownership is not supported only once

cmd/minify/main.go

@@ -383,6 +383,9 @@ func run() int {
 			preserveLinks = true
 		}
 	}
+	if preserveOwnership && !supportsGetOwnership {
+		Warning.Println(fmt.Errorf("preserve ownership not supported on platform"))
+	}
 
 	////////////////
 
@@ -960,8 +963,6 @@ Next:
 			if err != nil {
 				Warning.Println(err)
 			}
-		} else {
-			Warning.Println(fmt.Errorf("preserve ownership not supported on platform"))
 		}
 	}
 	if preserveTimestamps {

cmd/minify/syscall.go

@@ -4,6 +4,8 @@ package main
 
 import "os"
 
+var supportsGetOwnership = false
+
 func getOwnership(info os.FileInfo) (int, int, bool) {
 	return 0, 0, false
 }

cmd/minify/syscall_unix.go

@@ -7,6 +7,8 @@ import (
 	"syscall"
 )
 
+var supportsGetOwnership = true
+
 func getOwnership(info os.FileInfo) (int, int, bool) {
 	if stat_t, ok := info.Sys().(*syscall.Stat_t); ok {
 		return int(stat_t.Uid), int(stat_t.Gid), true