fix(ci): remove --sdk, add system sysroot paths for xlings gcc

The xlings gcc is built without system headers/CRT (like musl-gcc).
Unlike the xlings project which uses musl-gcc (self-contained libc),
mcpplibs-index needs system glibc. Fix by:

- Remove --sdk/--cc/--cxx flags (--sdk overrides sysroot)
- Put gcc/llvm bin dir on PATH for auto-detection
- Set LIBRARY_PATH, C_INCLUDE_PATH, CPLUS_INCLUDE_PATH to system
  paths so xlings gcc can find crt1.o, stdio.h, etc.

Author: Sunrisepeak

.github/workflows/ci.yml

@@ -91,27 +91,29 @@ jobs:
         working-directory: tests
         run: |
           GCC_VERSION=$(python3 -c "import json; d=json.load(open('../.xlings.json')); print(d['workspace']['gcc']['linux'])")
-          GCC_SDK="$HOME/.xlings/data/xpkgs/xim-x-gcc/${GCC_VERSION}"
-          echo "Using gcc ${GCC_VERSION} from ${GCC_SDK}"
-          "${GCC_SDK}/bin/gcc" --version | head -1
-          xmake f -P . -y -vv \
-            --sdk="${GCC_SDK}" \
-            --cc="${GCC_SDK}/bin/gcc" \
-            --cxx="${GCC_SDK}/bin/g++"
+          GCC_BIN="$HOME/.xlings/data/xpkgs/xim-x-gcc/${GCC_VERSION}/bin"
+          echo "Using gcc ${GCC_VERSION} from ${GCC_BIN}"
+          "${GCC_BIN}/gcc" --version | head -1
+
+          # xlings gcc needs system sysroot for headers and CRT files
+          export LIBRARY_PATH="/usr/lib/x86_64-linux-gnu:/usr/lib:/lib/x86_64-linux-gnu"
+          export C_INCLUDE_PATH="/usr/include:/usr/include/x86_64-linux-gnu"
+          export CPLUS_INCLUDE_PATH="/usr/include:/usr/include/x86_64-linux-gnu"
+          export PATH="${GCC_BIN}:$PATH"
+
+          xmake f -P . -y -vv
 
       - name: Configure (macos)
         if: runner.os == 'macOS'
         working-directory: tests
         run: |
           LLVM_VERSION=$(python3 -c "import json; d=json.load(open('../.xlings.json')); print(d['workspace']['llvm']['macosx'])")
-          LLVM_SDK="$HOME/.xlings/data/xpkgs/xim-x-llvm/${LLVM_VERSION}"
-          echo "Using llvm ${LLVM_VERSION} from ${LLVM_SDK}"
-          "${LLVM_SDK}/bin/clang" --version | head -1
-          xmake f -P . -y -vv \
-            --toolchain=llvm \
-            --sdk="${LLVM_SDK}" \
-            --cc="${LLVM_SDK}/bin/clang" \
-            --cxx="${LLVM_SDK}/bin/clang++"
+          LLVM_BIN="$HOME/.xlings/data/xpkgs/xim-x-llvm/${LLVM_VERSION}/bin"
+          echo "Using llvm ${LLVM_VERSION} from ${LLVM_BIN}"
+          "${LLVM_BIN}/clang" --version | head -1
+
+          export PATH="${LLVM_BIN}:$PATH"
+          xmake f -P . -y -vv --toolchain=llvm
 
       - name: Configure (windows)
         if: runner.os == 'Windows'
@@ -125,10 +127,13 @@ jobs:
           BUILD_ALL: ${{ needs.detect-changes.outputs.build_all }}
           CHANGED_PACKAGES: ${{ needs.detect-changes.outputs.packages }}
         run: |
-          # Put correct compiler on PATH for xmake build subprocesses
+          # Put correct compiler on PATH and set system paths for xmake subprocesses
           if [ "$(uname)" = "Linux" ]; then
             GCC_VERSION=$(python3 -c "import json; d=json.load(open('../.xlings.json')); print(d['workspace']['gcc']['linux'])")
             export PATH="$HOME/.xlings/data/xpkgs/xim-x-gcc/${GCC_VERSION}/bin:$PATH"
+            export LIBRARY_PATH="/usr/lib/x86_64-linux-gnu:/usr/lib:/lib/x86_64-linux-gnu"
+            export C_INCLUDE_PATH="/usr/include:/usr/include/x86_64-linux-gnu"
+            export CPLUS_INCLUDE_PATH="/usr/include:/usr/include/x86_64-linux-gnu"
           else
             LLVM_VERSION=$(python3 -c "import json; d=json.load(open('../.xlings.json')); print(d['workspace']['llvm']['macosx'])")
             export PATH="$HOME/.xlings/data/xpkgs/xim-x-llvm/${LLVM_VERSION}/bin:$PATH"