[bug #17881] Better documentation of make rules

[Paul D. Smith]

Follow-up Comment #2, bug #17881 (project make):

Actually, make _does_ guarantee that rules will be processed in left-to-right
order.  If you never use parallelism, you can be sure your rules will always
run in that order.

If you do use parallelism, though, obviously more than one of these rules
will run at the same time.  Even though make still starts them in
left-to-right order in the makefile, it's up to your operating system to
schedule the programs.  In this case it's almost certain that commands to
build the second target will start before the commands to build the first
have completed.

I'm sure you're aware of this.  However, saying the order is "insignificant"
is not exactly correct.  Saying that it is unreliable during parallel builds
is better.

As for Howard's suggestion, that suggestion has been made before and I think
it's a good one.  However, it's not completely trivial to implement.  For one
thing, we can't afford to change the values of variables such as $<, or even
$^ in some cases, so make would have to keep a separate list of "prereqs to
run" vs. "prereqs used in automatic variables".

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