region sensitive Makefile compile-command

[Dan Jacobson <jidanni@jidanni.org>]

You guys just got to implement this, so I'm rereqesting it as it is so
cool.  Make a new variable that we can turn on to enable this:

Here we are in a Makefile,
a:
        bla bla
b:
        bla bla

Well, if the cursor is anywhere in the a: region, all two lines of it,
then compile-command should be "make a" (or "make -k a"). If anywhere
in the b: region, then compile-command should be "make b".

That way when one hits M-x compile, one gets prompted with something
relevant, not just "make -k" which may not always be what you want.

Currently sadly, one can even put the cursor right on top of the a:
and normally highly sensitive emacs (especially after enabling ffap)
still doesn't get the hint.