add_jar: add option GENERATE_NATIVE_HEADERS

Tests/JavaNativeHeaders/D.java

@@ -0,0 +1,19 @@
+class D
+{
+  public D()
+    {
+    }
+
+    public native void printName();
+
+    static {
+        try {
+
+            System.loadLibrary("D");
+
+        } catch (UnsatisfiedLinkError e) {
+            System.err.println("Native code library failed to load.\n" + e);
+            System.exit(1);
+        }
+    }
+}