Sync clean_all_examples with make_all_examples

tools/clean_all_examples

 saved_pwd_before_making=$PWD
-for file in `find tools example -name CMakeLists.txt`; do
+for file in `find tools example -name CMakeFiles -type d -prune -o -name CMakeLists.txt -print`; do
     cd $(dirname $file)
     echo
     echo "[$file]"