Add Gradle build

Fixed warnings about missing deprecation annotation and variable name in
one test failing on Java greater version 9.

This commit is simpler then
5847ee4d97

settings.gradle

@@ -0,0 +1,10 @@
+/*
+ * This file was generated by the Gradle 'init' task.
+ *
+ * The settings file is used to specify which projects to include in your build.
+ *
+ * Detailed information about configuring a multi-project build in Gradle can be found
+ * in the user manual at https://docs.gradle.org/6.6.1/userguide/multi_project_builds.html
+ */
+
+rootProject.name = 'luaj'