RFR 8188023: Avoid -source and -target javac options in nashorn ant compilation

[Jim Laskey (Oracle)]

+1

> On Sep 27, 2017, at 8:59 AM, Sundararajan Athijegannathan <sundararajan.athijegannathan at oracle.com> wrote:
> 
> Please review.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8188023
> Webrev: http://cr.openjdk.java.net/~sundar/8188023/webrev.00/index.html
> 
> Piggybacking to take care of few javac warnings as well.
> 
> Thanks,
> -Sundar