acknowledge the deprecatio of set/unset in scala.concurrent.SyncVar

This is part one of two of the steps needed to make SBT 2.11 compatible.
The second step is porting the use of MurmurHash.scala in xsbt.HashApi to MurmurHash3.scala.

This PR is here to ensure the trivial parts (i.e. this) are not forgotten in the upcoming sbt release.