[m-rev.] for review: Fix bison and flex makefile rules in trace directory.

[Julien Fischer]

On Sat, 1 Jun 2019, Peter Wang wrote:

> I think this will fix the occasional build failures seen on the test
> server.
>
> ----
>
> trace/Mmakefile
>    Fix incorrect makefile rule with multiple targets.
>    A rule with multiple targets is equivalent to writing many rules,
>    each with one target, so the bison rule would (in parallel make)
>    run bison once for the .c target and once for the .h target.
>    The solution is to use a pattern rule, which GNU make recognises
>    as a single recipe generating multiple outputs.
>
>    Write separate rules for mercury_event_scanner.c and
>    mercury_event_scanner.h.

That's fine.

Julien.