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

[Sundararajan Athijegannathan]

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