Portable executables for Catala

[denismerigoux]

Context

For Catala to be usable by industrial users, it would be hugely beneficial if its distribution and installation would not entail installing the whole opam toolchain + the C/C++ dependencies : GMP, Z3.

First, as mentionned by https://github.com/CatalaLang/catala/issues/207, it would be good to be able to not include Z3 in the Catala compiler on demand with a custom build profile.

Portable executables

Second, an easy way to distribute the Catala compiler would be as a static, portable executable. @AltGr wrote this excellent tutorial about static binaries compiled from OCaml code: https://www.ocamlpro.com/fr/blog/2021_09_02_generating_static_and_portable_executables_with_ocaml. Following it, I was able to produce portable Linux/ELF x86-64 binaries for Catala and Clerk included in the v0.6.0 release.

First issue: z3 static linking

When creating these binaries, I had to use z3.4.8.11 instead of the latest z3.4.8.14 because z3.4.8.13 introduced a breaking change that forbids static linking:

https://github.com/ocaml/opam-repository/blob/1fbd9337269c59f74b4046cd9a60efeaa9e52d02/packages/z3/z3.4.8.13/opam#L32-L37

Can we overcome this ? The OCaml API readme in Z3's folder still seems to say that you can statically link https://github.com/Z3Prover/z3/tree/master/src/api/ml

Second issue: cross-compilation

Right now I managed only to compile for x86 ELF because that's what I got on my machine but we would need Windows and ARM/M1 binaries as well, preferably automatically generated through some CI that gets called at each release.

[denismerigoux]

Alternative cheap solution: a node JS binary... https://github.com/CatalaLang/catala/releases/download/0.6.0/catala_v_0_6_0_node.js

[denismerigoux]

Fixed by #299