Note that GNU make has received a Google Summer of Code project, to implement the ability for users to customize the "out of date" algorithm used by make.  You might consider waiting a couple of months and see what comes of that by the end of the summer.  I have high hopes that it will be flexible and powerful enough to provide all the functionality people need to implement various algorithms including things like comparing stored stat(2) info as well as .KEEP_STATE capabilities.

Please add a function $(mtime) that returns the modification time of a given file as the number of seconds since the Epoch.  This would be easy to do because make already caches mtimes.

I am writing a build tool on top of make, and I want it to have a better rule for whether a prerequisite makes a target out of date than make's.  Specifically, the tool will remember the prerequisite's mtime when the target was last built and consider the target out of date if the prerequisite's mtime has changed, forward or backward.  I want to implement this using a function in the target's prerequisite list that is called during second-expansion, checks the prerequisite's mtime, and expands to FORCE or nothing as appropriate.

The tool doesn't know which files' mtimes it is going to need until make actually gets to those files, so the tool can't easily do any better than getting the mtimes one at a time.  Getting each mtime currently requires running a stat(1) process, and doing this for every file checked (not just built) is intolerably slow.  The $(mtime) function would make it practical.