Merge topic 'cmState-definitions'

bff27391 cmState: Host variable definitions.
6954c893 cmState: Add a VariableScope snapshot type.
1fc645bd cmState: Add a Base snapshot type.
0f070dd3 cmMakefile: Decouple the container of cmDefinitions from scoping logic.
25e04ddf cmDefinitions: Implement in terms of cmLinkedTree.
4bbe261c cmMakefile: Extract InitializeVarScope method.