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                        MUT time (elapsed)       GC time  (elapsed)
  Task  0 (worker) :    0.00s    (  0.00s)       0.00s    (  0.00s)
  Task  1 (worker) :    0.00s    (  0.01s)       0.00s    (  0.00s)
  Task  2 (worker) :    0.00s    (  0.03s)       0.00s    (  0.00s)
  Task  3 (bound)  :    0.00s    (  0.00s)       0.02s    (  0.04s)

We still need to sort out the "install-docs" story to get this working
properly.

Now we have variables for whether or not to build the docbook docs as
HTML, as PS, and as PDF. The configure script output now matches what
the build system will do (except it cannot take account of any
mk/build.mk settings, of course).

We only need the forwardslash mode now, so always use that mode and
don't accept any arguments.

Also fixes it so that it really does pass (we weren't substituting the
GMP variables in the RTS package config).

configure looks for install-sh in . .. ../.. and we don't want it to
find the build system's install-sh.

No idea how the last patch managed to pass validate.

I didn't realise make can invoke the same action multiple
times in parallel if the rule has multiple targets. So go
back to using a build stamp file.

Unlike with static libs, when linking shared libs the dependencies
have to already exist. Add a dependency $1_$2_$3_LIBS : $1_$2_$3_DEPS_LIBS