convert multi-output recipes to pattern rules to avoid repetition

[bertsky]

This converts rules that specify multiple targets with a recipe that builds them all at once – as most multi-executable modules have – into pattern rules, so make "knows" that all the targets are built together. Thus, the rule only fires once per module (instead of once per executable), and parallel building (make -j) becomes possible again.

We could have waited for GNU make 4.2 for this, which finally added new syntax to support such rule semantics (via :& instead of :). But pattern rules can do this, too. They are just more difficult to read and write.

(I encapsulated this in a parameterized function multirule which takes the original list of executables and replaces the $(BIN)/ part – which is assumed to always be present – with %, so the rule becomes a pattern rule, but no other target is likely to match it. This is only necessary for the actual recipe, the dependencies still must be specified normally.)

[kba]

Elegant, less readable than :& but more portable and short. LGTM

[stweil]

Thank you.