The current version doesn't compile with:
ant jar-jme|all

The Java compiler complains with an obscure error:
    [javac] /Users/ducos/Projects/luaj/build/jme/src/org/luaj/vm2/LuaValue.java:532: error: cannot access StringBuilder
    [javac] 	public String  tojstring()           { return typename() + ": " + Integer.toHexString(hashCode()); }

This is because, in recent versions of Java, the concatenation of strings is automatically performed
with java.lang.StringBuilder

Unfortunately, this class is not available in the JME environment.

A workaround would be to replace the + concatenation with the String.concat() method, like this:

-       public String  tojstring()           { return typename() + ": " + Integer.toHexString(hashCode()); }
+       public String  tojstring()           { return typename().concat(": ").concat(Integer.toHexString(hashCode())); }

But it has to be done everywhere in the code.

Maybe there is a more straightforward method, with a compiler's flag.