[m-rev.] for review: keep the invocation automatically up-to-date

[Zoltan Somogyi]

For review by anyone.

The main issue I am seeking feedback on is that after this diff,
making the user guide requires compiler/mercury_compile to be
up to date. The other rules of doc/Mmakefile already depend
on the contents of the compiler directory, but they do so only for
.m files, which do not need to be built.

Note that the top level makefile builds the doc directory *after*
the compiler directory, so this won't be an issue in the vast majority
of cases, but maybe one of you knows an atypical case where it may
cause an issue.

Zoltan.
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