Current patch

[Ramón García]

Here is my current patch. It implements persistency, based on the
proposal by Paul, showing the previously stored value as $= to the
.OUT_OF_DATE expression.

I see some practical issues. I guess persistency should not be enabled
by default, only for rules that request it. For instance, something
like

target: .PERSISTENT=1

As already mentioned, is that downward rule propagation is
problematic. It should be posible to restrict it.

The representation of the state as a single variable, ("$=" in the
current implementation, as suggested) might be troublesome for users
writting user-defined out of date determination. For instance, how
would one write an .OUT_OF_DATE expression that uses MD5 sums? Even if
a builtin is provided, this could be a test of generality. As an
example of the need of generality, consider the case of the Java
programming language, where changes in a source .java file may require
or not recompiling files that reference it, depending on whether the
change in the java source file changes the implementation only or
changes the interface as well.

Ramon