reproducible engine binary build

[bmorel]

Currently, generated binary files contains the absolute path from where they where built. This is unnecessary, break reproducible builds and can leak informations about modder's systems.

I think this should be fixed. This also applies to unvanquished.

[necessarily-equal]

For the record, the NixOS package (that builds the engine) is reproducible. Likely because the build happens in a fixed directory.

$ nix-build '<nixpkgs>' -A unvanquished --check --keep-failed
/nix/store/vn4y8yiijl37qvmr2zvgpgdywhw235w8-unvanquished-0.52.1
(exit 0)