RFR: JDK-8062661 Do not perform X11 checks in configure when X11 is not needed

[Tim Bell]

Magnus:

> Configure has been testing for X11 even though it was not needed (on 
> macosx and windows). This never caused any failures (since we ignore 
> any results indicating that X11 is missing), but it looks silly.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8062661
> WebRev: 
> http://cr.openjdk.java.net/~ihse/JDK-8062661-dont-check-X11-if-not-needed/webrev.01

Looks good to me.

Tim