Fix native CI: use leanc for ABI-compatible FFI build

Lean's linker uses its own sysroot + libc++. Building the FFI library
with system GCC (libstdc++) causes ABI mismatches (__libc_csu_fini,
std::__cxx11::basic_string undefined symbols).

- CI: use leanc as cmake compiler with STATIC_LIBRARY try-compile
- lakefile.lean: remove -lstdc++/-lgcc_s, add -lzmq for system ZMQ

Author: junjihashimoto

.github/workflows/ci.yml

@@ -38,7 +38,13 @@ jobs:
 
     - name: Build C++ FFI library
       run: |
-        cmake -S . -B build-cmake
+        # Use leanc (Lean's clang wrapper) so the FFI library uses libc++,
+        # matching Lean's linker ABI. STATIC_LIBRARY skips cmake link tests
+        # that fail with leanc's custom sysroot.
+        cmake -S . -B build-cmake \
+          -DCMAKE_C_COMPILER=leanc \
+          -DCMAKE_CXX_COMPILER=leanc \
+          -DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY
         cmake --build build-cmake
 
     - name: Build Lean kernel

lakefile.lean

@@ -42,15 +42,18 @@ lean_exe xlean where
         "-Wl,-rpath,@executable_path/../../../build-cmake/_deps/xeus-zmq-build",
         "-lxeus", "-lxeus-zmq", "-lstdc++"]
     else  -- Linux
+      -- FFI library must be built with leanc (Lean's clang/libc++) for ABI
+      -- compatibility. Lean's linker uses its own sysroot and libc++, so
+      -- libstdc++ and GCC runtime are not needed.
       #["-Wl,--start-group",
         "./build-cmake/libxeus_ffi.a",
         "-L./build-cmake/_deps/xeus-build",
         "-L./build-cmake/_deps/xeus-zmq-build",
-        "-lxeus", "-lxeus-zmq",
+        "-L./build-cmake/_deps/libzmq-build/lib",
+        "-lxeus", "-lxeus-zmq", "-lzmq",
         "-Wl,--end-group",
-        "-L/usr/lib/gcc/x86_64-linux-gnu/13",
         "-L/usr/lib/x86_64-linux-gnu",
-        "-lstdc++", "-lgcc_s", "-lpthread", "-lm", "-ldl"]
+        "-lpthread", "-lm", "-ldl"]
 
 /-- Script to build xlean via cmake -/
 script buildXlean do