Preliminary compatibility with JDK 9

[retronym]

This pull request cleans up two small issues that stood in the way of using the soon-to-be-released Java 9 with SBT.

• No more java.ext.dirs
• java.xml.bind not part of the java.base default classpath

Scala compiler versions prior to 2.12.1 do not tolerate the absence of rt.jar from the JDK. But we can appease them a little manual setup.

First, we can extract an equivalent JAR with a tool I've written:

% git clone https://github.com/retronym/java9-rt-export
% cd java9-rt-export/
% java_use 1.8
% sbt package
% java_use 9
% mkdir -p $HOME/.sbt/0.13/java9-rt-ext; java -jar target/java9-rt-export-*.jar $HOME/.sbt/0.13/java9-rt-ext/rt.jar
% jar tf $HOME/.sbt/0.13/java9-rt-ext/rt.jar | grep java/lang/Object

Then, we can feed that to the compilation classpath of Scala compilers used within SBT with the scala.ext.dirs System property.

% java_use 9
% sbt -Dscala.ext.dirs=$HOME/.sbt/0.13/java9-rt-ext clean compile console
[info] Loading global plugins from /Users/jason/.sbt/0.13/plugins
[info] Compiling 1 Scala source to /Users/jason/.sbt/0.13/plugins/target/scala-2.10/sbt-0.13/classes...
[info] Loading project definition from /Users/jason/code/scratch1/project
[info] Set current project to scratch1 (in build file:/Users/jason/code/scratch1/)
[info] Defining *:scalaVersion
[info] The new value will be used by *:allDependencies, *:crossScalaVersions and 13 others.
[info] 	Run `last` for details.
[info] Reapplying settings...
[info] Set current project to scratch1 (in build file:/Users/jason/code/scratch1/)
[success] Total time: 0 s, completed 4 Feb. 2017, 10:37:59 pm
[info] Updating {file:/Users/jason/code/scratch1/}scratch1...
[info] Resolving jline#jline;2.12.1 ...
[info] Done updating.
[info] Compiling 1 Scala source and 1 Java source to /Users/jason/code/scratch1/target/scala-2.11/classes...
[info] 'compiler-interface' not yet compiled for Scala 2.11.8. Compiling...
[info]   Compilation completed in 7.315 s
[success] Total time: 9 s, completed 4 Feb. 2017, 10:38:08 pm
[info] Compiling 1 Scala source to /Users/jason/code/scratch1/target/scala-2.11/classes...
[info] Starting scala interpreter...
[info]
Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 9-ea).
Type in expressions for evaluation. Or try :help.

scala> java.lang.invoke.VarHandle.fullFence _
res0: () => Unit = <function0>

Note that this doesn't actually require you to use Scala 2.12.1 in your application, 2.11.8 is able to use Java 9.

Also note that scalac does not yet enforce the stricter module accessibility rules imposed by JEP-261, so your code might typecheck and compile but incur LinkageError-s at runtime if you are calling non-exported APIs.

[dwijnand]

Notes in #2952

[eed3si9n]

We need to forward port these.