* Update cxxstd values for GHA msvc jobs
* Add msvc support for [[maybe_unused]] attribute.
Fixes: https://github.com/boostorg/serialization/issues/271
Also removes failing cuda+clang test as it's an inscrutable system setup issue.
* Add C++23 header macros.
* Minor corrections.
* Remove XCode-9.4 testing as it appears to be no longer supported.
* Update docs for C++23 macros.
[CI SKIP]
* Add C++23 feature test macro support.
* Use __apple_build_version__ to detect Apple clang
…since __APPLE__ is defined by both Apple Clang and LLVM.org Clang
on Apple platforms.
* Update BOOST_CLANG_VERSION for Xcode 14.3, 15.0, 15.1
* Update CI clang version now that gcc has changed. (#489)
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Co-authored-by: Peter Dimov <pdimov@gmail.com>
Co-authored-by: Christopher Chavez <chrischavez@gmx.us>
