boost: fix cross-compile support

[oxalica]

Motivation for this change

Fixes cross-compile support for boost. Fixes #96336.

This PR is just to speed up #105478 since it is stalled for months. Changes requested are applied.

Tested to be built successfully: (on 8d46a65f63226c50d3c73ea309e7e35453c7d5fc)

• boost{15x,16x,17x} (not changed)
• pkgsCross.aarch64-multiplatform.boost{15x,16x,17x}
• pkgsCross.riscv64.boost17x

Related: #101651
Closes: #105478

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

cc @hlandau @veprbl @r-burns

[Mic92]

@Mic92 Oops, sorry for it. I should rebase before switching merge target... Now rebased.

You can also write rebase staging with a leading / to make a bot do the rebase.

[r-burns]

I thought only committers can do the rebase bot command, no?

[oxalica]

@veprbl

... or have --with-bjam=${boost-build}/bin/b2

I just realized that there is already boost-build in nixpkgs. I tried to upgrade it and it successfully builds boost.
I prefer this approach, but it will cause a mass rebuild once boost-build is upgraded (if a new boost release requires a new boost-build).

Otherwise, if we want to minimize rebuilds when adding a new boost version, maybe we should apply this:

+ sed '2iexport CXX=$CXX_FOR_BUILD' -i ./tools/build/src/engine/build.sh

What do you think?

[veprbl]

Supporting build with external boost-build may put extra burden on maintainers, so I, personally, would not call for that. I think it would only be viable if we extracted the exact boost-build version that corresponds to each given boost version (e.g. by looking at the submodule versions at https://github.com/boostorg/boost/tree/master/tools). That should prevent possible incompatibilities.

[oxalica]

I changed to patch ./tools/build/src/engine/build.sh instead, to keep using in-tree boost-build.
The reason why I use sed instead of add a patch is to avoid multiple version-specific patches required.

If this is acceptable, we can rebase it back to master, since it doesn't break non-cross build anymore.

[veprbl]

/rebase master

[github-actions]

Rebased, please reopen the pull request to restart CI

[Mic92]

Not tested but change looks minimal invasive.