LLVM 15 regression: LIBCXX_STATICALLY_LINK_ABI_IN_STATIC_LIBRARY does not work with system-libcxxabi

[nikic]

When doing a build with -DLIBCXX_CXX_ABI=system-libcxxabi -DLIBCXX_ENABLE_STATIC=ON -DLIBCXX_STATICALLY_LINK_ABI_IN_STATIC_LIBRARY=ON the resulting libc++.a will not in fact have libc++abi.a linked in (without producing an error).

This is because

llvm-project/libcxx/src/CMakeLists.txt

Line 313 in e488ce2

target_link_libraries(cxx_static PRIVATE libcxx-abi-static-objects)

links against libcxx-abi-static-objects, which is not defined when building against anything but the bundled libcxxabi.