RFR: JDK-8139413 Use --with-x to set X11 root directory

[Magnus Ihse Bursie]

Similarly to the other libraries, we should use --with-x as a 
convenience argument so set the root of the X11 directory, and assume 
that includes and libraries are found in "include" and "lib" 
respectively, in this directory.

Bug: https://bugs.openjdk.java.net/browse/JDK-8139413
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8139413-use-with-x-to-set-X11-root/webrev.01

/Magnus