handle no lxc installed

libexec/gconfig

@@ -1,9 +1,11 @@
 VM_SSH_PORT=2223
-if [ -z "$LXC_EXECUTE" ]; then
-  ver=`lxc-start --version`
-  if dpkg --compare-versions $ver ge 1.0.0 ; then
-    LXC_EXECUTE=lxc-execute
-  else
-    LXC_EXECUTE=lxc-start
+if [ -n "$USE_LXC" ]; then
+  if [ -z "$LXC_EXECUTE" ]; then
+    ver=`lxc-start --version`
+    if dpkg --compare-versions $ver ge 1.0.0 ; then
+      LXC_EXECUTE=lxc-execute
+    else
+      LXC_EXECUTE=lxc-start
+    fi
   fi
 fi