ci: remove gcc-14 and add clang-14 with gcc-11 test

- Add clang-14-libstdcxx-gcc11 matrix entry for testing
- Remove gcc-14 to prevent Clang from finding it
- Add diagnostic to show which headers Clang will use
- Test both gcc-11 and gcc-13 with clang-14

.github/workflows/xahau-ga-nix.yml

@@ -24,7 +24,7 @@ jobs:
         # - cc/cxx: Actual compiler executables
         # - compiler_version: Version number for Conan
         # - stdlib: Standard library (libstdcxx/libcxx) for Conan/CMake configuration
-        compiler_id: [clang-14-libstdcxx-gcc13, clang-16-libstdcxx-gcc13]  # Temporarily disabled: gcc-13-libstdcxx
+        compiler_id: [clang-14-libstdcxx-gcc11, clang-14-libstdcxx-gcc13, clang-16-libstdcxx-gcc13]  # Temporarily disabled: gcc-13-libstdcxx
         configuration: [Debug]
         include:
           # - compiler_id: gcc-13-libstdcxx
@@ -33,6 +33,13 @@ jobs:
           #   cxx: g++-13
           #   compiler_version: 13
           #   stdlib: libstdcxx
+          - compiler_id: clang-14-libstdcxx-gcc11
+            compiler: clang
+            cc: clang-14
+            cxx: clang++-14
+            compiler_version: 14
+            stdlib: libstdcxx  # Required for clang-14 compatibility
+            clang_gcc_toolchain: 11  # Use GCC 11 headers with Clang 14
           - compiler_id: clang-14-libstdcxx-gcc13
             compiler: clang
             cc: clang-14
@@ -66,6 +73,10 @@ jobs:
             echo "Installing GCC ${{ matrix.clang_gcc_toolchain }} toolchain for Clang"
             sudo apt-get install -y gcc-${{ matrix.clang_gcc_toolchain }} g++-${{ matrix.clang_gcc_toolchain }}
             
+            # Remove GCC 14 to prevent Clang from finding it
+            echo "Removing GCC 14 to prevent conflicts"
+            sudo apt-get remove -y gcc-14 g++-14 || true
+            
             # Set the specified GCC version as the system default using update-alternatives
             echo "Setting GCC ${{ matrix.clang_gcc_toolchain }} as the default compiler"
             sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-${{ matrix.clang_gcc_toolchain }} 100
@@ -76,6 +87,10 @@ jobs:
             # Verify the change
             gcc --version
             g++ --version
+            
+            # Show what Clang will use
+            echo "Checking what headers Clang will use:"
+            clang++ -E -x c++ - -v < /dev/null 2>&1 | grep "^ /"
           fi
           
           # Install libc++ dev packages if using libc++ (not needed for libstdc++)