Fix the target all of the Makefile

[Halbaroth]

The PR #887 was wrong... The target @check of Dune doesn't build all the binaries, it builds only binary artefacts cmi, cmx, ... but no linkage is performed.

This commit mostly reverts the changes. As the sources are located in src/, I use the target @src/all. Now the command make will build everything in the directory src. I also add examples since we want to check the libusage module still compile.

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I also tried two other ways to fix the situation. I discover the stanza enabled_if for Dune's tests. We can create a profile or use an environment variable to enable or disable tests. For instance, we could have an environment variable DISABLE_TESTS whose the default value would be false and the all target of the Makefile would be DISABLE_TESTS=true dune build.

Note: the new all target copies the binary of AE at the root of the project as the target bin does. In my opinion, using dune exec -- alt-ergo ... is better since it prevents to perform tests on the wrong binary, but I remember @bclement-ocp had a good argument to keep copying the binary (something about the option --version of AE?)

[bclement-ocp]

Ah, dang, yeah, let's revert for now.

For the other points:

• Using dune exec is good for local tests but makes dune check for changes each time; it shouldn't be used in a tight loop. That said, copying the binary to ./alt-ergo was done for backwards compatibility (we do promote to src/bin/Main_text.exe using a (promote (until-clean)) rule; this must be kept because Marvin/Shepherd uses that).
• The version stuff no longer matters since we explicitly set (version dev) in dune-project.
• Instead of using enabled_if, we should redefine the default alias in dune to avoid building tests, see here.