change extension to custom

src/SPC/builder/extension/gd.php

@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace SPC\builder\extension;
+
+use SPC\builder\Extension;
+use SPC\util\CustomExt;
+
+#[CustomExt('gd')]
+class gd extends Extension
+{
+    public function getUnixConfigureArg(): string
+    {
+        $arg = '--enable-gd';
+        if ($this->builder->getLib('freetype')) {
+            $arg .= ' --with-freetype ' .
+                'FREETYPE2_CFLAGS=-I"' . BUILD_INCLUDE_PATH . '/freetype2" ' .
+                'FREETYPE2_LIBS="' . $this->getLibFilesString() . '"';
+        }
+        $arg .= ' PNG_CFLAGS=-I"' . BUILD_INCLUDE_PATH . '" ' .
+            'PNG_LIBS="' . $this->getLibFilesString() . '"';
+        return $arg;
+    }
+}