Can't test whether it works (no current >=mysql-4.1.2 available on Windows), but it should.

ext/mysqli/config.w32

@@ -0,0 +1,14 @@
+// $Id$
+// vim:ft=javascript
+
+ARG_WITH("mysqli", "MySQLi support", "no");
+
+if (PHP_MYSQLI != "no") {
+	if (CHECK_LIB("libmysql.lib", "mysql", PHP_MYSQL) &&
+			CHECK_HEADER_ADD_INCLUDE("mysql.h", "CFLAGS_MYSQL", PHP_PHP_BUILD + "\\include\\mysql;" + PHP_MYSQL)) {
+		EXTENSION("mysql", "mysqli.c mysqli_api.c mysqli_fe.c mysqli_nonapi.c mysqli_prop.c mysqli_report.c");
+		AC_DEFINE('HAVE_MYSQLILIB', 1, 'Have MySQLi library');
+	} else {
+		WARNING("mysqli not enabled; libraries and headers not found");
+	}
+}