CMake: Add customizable target names for compression libraries

[tarun-t]

• Introduce CACHE variables for ZLIB, BZip2, LibLZMA, and Zstd targets
• Update boost_iostreams_option function calls to use new variables

[tarun-t]

@pdimov @mclow Drawing your attention to this PR. I am happy to do it any other way if required.

[pdimov]

You probably need to tell us what's the purpose of this change.

[tarun-t]

This change introduces configurable target names for compression libraries used by Boost.Iostreams. The main driver is to allow static linking with zstd, which isn't possible with the current hardcoded zstd::libzstd_shared target. By making target names user-configurable via cache variables, we provide flexibility for different build environments and library configurations without breaking existing setups.

[pdimov]

Looks like this has already been proposed in #173.

So, the only use case is changing the zstd target name from zstd::libstd_shared to zstd::libstd_static? In that case, the rest of the target variables should be removed, and I think we also need to list the values of BOOST_IOSTREAMS_ZSTD_TARGET.

Note that these target names are referenced here

https:/boostorg/cmake/blob/bb741d09d2cb12f3934896880c6ecaf6de4ba707/include/BoostInstall.cmake#L367-L381

so that the installed config files add the appropriate find_dependency call. It looks like ztsd::libstd_static is already handled there because of boostorg/cmake#59.

[tarun-t]

Thank you for the feedback, @pdimov. I've updated the PR based on your suggestions:

• Simplified the changes to focus on the zstd target name
• Added an explicit list of possible values for BOOST_IOSTREAMS_TARGET_ZSTD
• Implemented error checking to ensure only valid values are used

This change allows users to choose between shared and static zstd libraries by setting BOOST_IOSTREAMS_TARGET_ZSTD, with proper validation to prevent incorrect values.

Please let me know if you need any further modifications or clarifications.