[BUG]: finer grained ABI compatibility

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Problem description

Based on the extensive discussion in conda-forge plus offline discussion with @isuruf, we think there may be a better approach to ABI compatibility with respect to compilers and c++ runtimes.

Currently, pybind11 uses a combination of various things. The one of interest here is the __GXX_ABI_VERSION. This appears to be too strict, especially for forward-compatible libraries commonly in use.

The suggestion is to replace __GXX_ABI_VERSION with

• __GLIBCXX_USE_CXX11_ABI
• __GLIBCXX_USE_DEPRECATED
• __SEH__ (only in clang I think)
• __SJLJ_EXCEPTIONS__

These capture the fine-grained, core ABI changes without being overly prescriptive. The last two are related to exception handling. There may be more macros to use to capture dwarf2 exceptions explicitly.

Thoughts @henryiii @isuruf?

Reproducible example code

No response

The issue does not name files, tests, or provide a reproducible example. Start by locating pybind11's current use of __GXX_ABI_VERSION, then compare that logic with the proposed __GLIBCXX_USE_CXX11_ABI, __GLIBCXX_USE_DEPRECATED, SEH, and SJLJ_EXCEPTIONS macros. Done would require an agreed ABI-compatibility design and corresponding validation.