第6组：启用ccache加速重复编译并保持闭环

test-linuxcnc-source-syntax.sh

@@ -5,6 +5,9 @@ cd "$(dirname "$0")"
 
 linuxcnc_root=${LINUXCNC_ROOT:-../linuxcnc}
 cxx=${CXX:-g++}
+if [[ -z ${CXX:-} ]] && command -v ccache >/dev/null 2>&1; then
+    cxx="ccache $cxx"
+fi
 build_jobs=${CNC_SIM_BUILD_JOBS:-8}
 if ! [[ "$build_jobs" =~ ^[1-9][0-9]*$ ]]; then
     echo "CNC_SIM_BUILD_JOBS must be a positive integer: $build_jobs" >&2