3.1.6 .KEEP_STATE and Special Dependency Checking

Use the special target .KEEP_STATE to check for command dependencies and hidden dependencies.

When the .KEEP_STATE: target is effective, make checks the command for building a target against the state file. If the command has changed since the last make run, make rebuilds the target.

When the .KEEP_STATE: target is effective, make reads reports from cpp(1) and other compilation processors for any "hidden" files, such as #include files. If the target is out of date with respect to any of these files, make rebuilds it.