Refactor cmFileTimeComparison and rename to cmFileTimeCache
This
- Adds an OS independent
cmFileTime
class that loads and stores a file modification time in nanoseconds since a common epoch. - Refactors
cmFileTimeComparison
to usecmFileTime
internally - Renames
cmFileTimeComparison
tocmFileTimeCache
- Renames
cmFileTimeCache
class methodsFileTimesDiffer
andFileTimeCompare
- Renames variables
- Adds file time one time computation optimizations