Remove the hard dependency on boost::system compatibility library to fix builds with Boost 1.89.0

[cho-m]

Boost.System has been header-only since Boost 1.69¹ and will be dropping the compatibility stub library in Boost 1.89 (boostorg/system@7a495bb).

Since the system component is only needed for Boost < 1.69, it can be added to BOOST_COMPONENTS in the fallback find_package.

Footnotes

1. https://www.boost.org/doc/libs/1_69_0/libs/system/doc/html/system.html#changes_in_boost_1_69 ↩

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[clonker]

Thank you for getting this rolling! I think the usage of the Boost_SYSTEM_LIBRARY is not optimal and I'd rather not rely on it

[cameel]

Since the system component is only needed for Boost < 1.69, it can be added to BOOST_COMPONENTS in the fallback find_package.

I'd recommend just bumping minimum Boost version to 1.69 and removing the fallback for simplicity. We don't have all a strong reason to support such old versions, and it's very likely that we'll have to bump minimum Boost version to 1.75 anyway due to #16084.

[cameel]

BTW, we don't use boost::system too extensively. Just in a couple of test helpers to check error codes when deleting files or creating symlinks. We could probably easily replace those with std::filesystem equivalents and drop the usage of the library completely.

[clonker]

Since the system component is only needed for Boost < 1.69, it can be added to BOOST_COMPONENTS in the fallback find_package.

I'd recommend just bumping minimum Boost version to 1.69 and removing the fallback for simplicity. We don't have all a strong reason to support such old versions, and it's very likely that we'll have to bump minimum Boost version to 1.75 anyway due to #16084.

Fair point. We can do that in a follow-up and think about bumping it to 1.70, then it always comes with a cmake config and we can leave out the entire branch:

https://github.com/ethereum/solidity/blob/c79f2ccda62e1a6871f9cef6a54770b64c83acc0/cmake/EthDependencies.cmake#L47-L54