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See !5259

The build system now does this automatically.

Split make build system documentation out of root README and reference
shake-based build system.

This adds the port of the smallpt raytracer by David Scies and bollu.

https://github.com/bollu/smallpt-opt/

The rule did not produce this file.

We rather generate `Main.cachegrind.result.<n>`. For now I'll just let
this fact go unrepresented in the build graph.

To avoid interfering with test output.

This introduces a Shake-based build system, building on Neil Mitchell's
prototype from several years ago [1].

This build system has a number of features relative to the existing
make-based build system:

 * results are produced in a common CSV structure, making it easy to use
   the included analysis tool nofib-compare, to compare results from tests
   outside of nofib.
 * Cachegrind runs can be run in parallel, greatly speeding up such
   measurements
 * native support for (micro-)architectural performance counters on
   Linux via `perf stat`

[1] https://gist.github.com/ndmitchell/4733855

There are also a few more changes to tests to make this build system
work well with nofib.

In particular:

* The Main module for some benchmarks has been renamed.
* The boot phase of benchmarks is now done by a boot.sh script.
* Replace some uses of perl with ghci.
* Some benchmarks that we didn't use to run by default had
  missing/wrong(!) stdout files. I added/updated these.

Take note that shake will run *more* benchmarks by default than make
used to. So you might want to select a subset of benchmarks to run when
using it.

Improve consistency across tests.

Previously I would get an error during `make boot`:

    EffBench.hs:10:18: error:
        Could not load module ‘Control.Monad.State.Strict’
        It is a member of the hidden package ‘mtl-2.2.2’.
        You can run ‘:set -package mtl’ to expose it.
        (Note: this unloads all the modules in the current scope.)
        Use -v (or `:set -v` in ghci) to see a list of the files searched for.
       |
    10 | import qualified Control.Monad.State.Strict as S
       |

Since morally these flags are for the program and not runtest

Remove both unnecessary SRC_DEPS depenencies and -fglasgow-exts flags.