Use /opt/boost as installation target for osx CI

[clonker]

This fixes the warnings we have seen earlier in #15982 so that the PEDANTIC=OFF switch can be removed again.

[blishko]

I think using a different installation prefix is a good solution.

By the way, what is the reason Boost has to be compiled from source and the version from homebrew is not sufficient?

[clonker]

I think this was initially done because we also draw our binary releases from this and there have been slight and subtle breaking changes (e.g. the filesystem handling) in the past when changing Boost versions. For our binaries it doesn't matter so much because it's all statically linked together and there is no visible boost dependency anymore. It does matter for homebrew (or arch linux when using the aur package), as they build against the latest and therefore you'd see these little differences there. OTOH, they are not official releases of ours and should be thoroughly tested before used in production.