Stephen Kelly authored May 24, 2015 and Brad King committed May 27, 2015

Refactor the local generator creation API to accept a
cmState::Snapshot.  Adjust MakeLocalGenerator to use the 'current'
snapshot in cases where there is no parent.  Create the snapshot
for subdirectories in cmMakefile::AddSubdirectory.

This means that snapshots are now created at the point of extending the tree,
as appropriate, and independently of the cmLocalGenerator and cmMakefile they
represent the state for.