RFR: JDK-8078437: Enable use of devkits for Windows

[Erik Joelsson]

Hello,

Internally at Oracle, we have been using devkits for building on Linux 
and Solaris. We would like to extend this to also work on Windows for 
our internal build servers. Using a devkit simplifies build server setup 
and maintenance greatly by not having to pre install a certain version 
of Visual Studio but instead being able to just plop down a bundle with 
the correct version.

This patch provides a script that generates the devkit for VS2013 (for 
future reference and documentation) and changes configure to accept it 
using the --with-devkit option.

Configuring a build to use the installed Visual Studio will continue to 
work as before and will most likely continue to be the preferred way of 
building for most developers.

Bug: https://bugs.openjdk.java.net/browse/JDK-8078437
Webrev: http://cr.openjdk.java.net/~erikj/8078437/webrev.01/

/Erik