A small change that make the project compilable with c++11

This change was necessary for my configuration to compile the project using c++11 instead of c++98 using the command:
./configure CXX='g++ -std=c++11'
A speed improvement with c++11 may be expected thanks to the the new move constructor semantic introduced by the language. This should indeed save some memory allocation time for example when a string object is returned by the function. I didn't compare the execution time with a binary produced with c++98.