Make sure that the opt and byte targets can be built concurrently.
The main issue is that both ocamlc and ocamlopt write a .cmi file when there is no .mli file, so the .cmi file might end up being corrupted due to concurrent accesses. The ugly fix is to prevent ocamlopt from creating a .cmi file by lying to it: ".mli files are no interface files, .but cmi files are". So ocamlopt behaves as if there had been a .mli file in the first place and does not try to produce a compiled interface from the .ml file. Note that we use the .cmi file because it happens to have the same base name, but any other file would have worked, e.g. the .dep file, since ocamlopt does not even read the file to check that it is actually an interface file. Since ocamlopt now believes that the .ml file has an interface, it needs a compiled interface to check its signature against, so the .cmi file has to be created beforehand. So the .cmx file is made to depend on the .cmi file and the .cmi file on the .cmo file. This might seem to add a dependency from the opt build on the byte build, but since ocamldep already says that the tools depend on lib/why3/why3.cmo due to the pack, this does not make the situation any worse. Note that there is a single file for which the .cmi hack cannot be used: src/tools/why3extract.ml. Indeed, if we lie to ocamlopt, it will try to use lib/why3/why3extract.cmi as the compiled interface (?!) and thus fail to compile src/tools/why3extract.ml, obviously. So the hack is disabled for this file, which means that src/tools/why3extract.cmi could end up being corrupted, but we do not care.
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