Revise libsingular_jll handling

[fingolfin]

• we probably should specify the version in Project.toml via =VERSION to pin a specific version, thus avoiding issues caused by Singular.jl and libsingular-julia not being in sync
• the code for re-compiling the libsingular code should be disabled when Singular.jl is not dev'ed

[fingolfin]

To elaborate:

We recently had trouble because we update the libsingular_julia_jll patch version, but did not immediately update Singular.jl. As a result people who did Pkg.update() got the new libsingular_julia_jll but with an older Singular.jl that did not expect it. As a result, Singular.jl decided to compile its own version of libsingular_julia_jll which however failed for people who don't have cmake installed. As a result, for these developers Oscar.jl was broken: bad.

I see these options to address it (note that they could also be combined)

1. lock the version of libsingular_julia_jll in Project.toml to an exact match, i.e., =VERSION in the compat section. This avoids the problem completely, but has the drawback that we can't update libsingular_julia_jll seperate from Singular.jl -- though IMHO that's a minor loss
2. modify the code which tries to recompile libsingular_julia_jll. Some alternatives for that:
   1. only run it when Singular.jl has been "dev'ed"
   2. only run it when cmake is present
   3. only run it when the version of libsingular_julia_jll is <= the version in the compat (i.e. assume for a "too new" libsingular_julia_jll that it is intentionally different)

[benlorenz]

Regarding option 1: I just did exactly such an update for libpolymake_julia, rebuilding it (with a new version due to the compat) to adapt to the new libcxxwrap_julia version for julia nightly (JuliaPackaging/Yggdrasil#7725). This would not be possible anymore without a Singular.jl release. Edit: Similarly for rebuilding against a new Singular_jll version.

I think it would be reasonable to have the automatic compilation opt-in only, i.e. if whatever conditions are met (e.g. dev + cmake + ...) print a warning and suggest setting some environment variable / Preferences.jl entry. Only if this is set run the automatic compilation.

[ederc]

I support to do the exact match of libsingular_julia_jll in Project.toml, any update should give us at least a minor release of Singular.jl. IMHO this would help speeding up the workflow for people working on Singular stuff in Oscar.jl.