change extension to custom

src/SPC/builder/extension/zstd.php

@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+namespace SPC\builder\extension;
+
+use SPC\builder\Extension;
+use SPC\util\CustomExt;
+
+#[CustomExt('zstd')]
+class zstd extends Extension
+{
+    public function getUnixConfigureArg(): string
+    {
+        return '--with-libzstd';
+    }
+}