Add REPL exports command backed by optional LIEF integration

[YSaxon]

Motivation

• Provide an easy REPL command to list exported symbols/functions from a shared library in a cross-platform, cross-architecture way using LIEF when available.
• Integrate LIEF consistently with existing dependency patterns so cross-target CI (Conan/CMakeDeps / find_package) can supply LIEF across architectures while allowing local builds to remain optional.

Description

• Add a new LIEF-backed module src/export_symbols.cpp and header src/export_symbols.h that expose list_exported_symbols_with_lief() and print exported symbol names when CLIFFI_HAS_LIEF is enabled.
• Add a new REPL command parser parseListExports() and wire exports <library> into src/main.c (help text, command routing, and error handling) to call the new module.
• Update CMakeLists.txt to enable C++ compilation, add export_symbols.cpp to sources, and detect/link LIEF via pkg-config or find_package(lief) depending on the build flow; expose CLIFFI_HAS_LIEF compile-time definition when found.
• Add lief/0.15.1 to conanfile.txt, update README.md docs, Homebrew formula cliffi.rb, and macOS CI steps to include LIEF so cross-arch CI can supply the dependency.
• Provide a graceful runtime fallback: if LIEF is not available in the build, exports prints a clear error instructing users to rebuild with LIEF support.

Testing

• Configured and built with cmake -S . -B build -DCMAKE_BUILD_TYPE=Release and cmake --build build -j4 successfully.
• Ran the full test suite with ctest --output-on-failure and all unit/integration tests passed (156/156 tests passed).
• Manually exercised the new REPL command with a local build lacking LIEF: printf 'exports ./build/libcliffi_test.so\nexit\n' | ./build/cliffi --repl and observed the graceful fallback error message (expected behavior).
• Attempted conan install to validate Conan/CMakeDeps flow but conan was not available in the environment, so cross-arch package validation was not executed locally.

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[chatgpt-codex-connector]

Set a minimum C++ standard for the LIEF codepath

This new implementation uses C++11 features (std::unique_ptr and range-based iteration), but the build files do not set CMAKE_CXX_STANDARD (or target-level CXX_STANDARD), so builds that enable LIEF on toolchains whose default is pre-C++11 will fail at compile time. This is especially risky for older cross-compilers in the dockcross/conan workflow where defaults vary by image.

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