Fixed build warning with gcc 4.7.3

  [Problem] build warning with gcc 4.7.3
            threads/main.cc:84:31: warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
            threads/list.cc:144:73: warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
[Reference] http://stackoverflow.com/questions/59670/how-to-get-rid-of-deprecated-conversion-from-string-constant-to-char-warnin
            http://stackoverflow.com/questions/7468286/warning-deprecated-conversion-from-string-constant-to-char