Fix error in Swarm::erase()

- The errro is due to a different declaration of the function
  std::list::erase(), in msvc erase() receives a const_iterator,
  whereas in gcc it receives an iterator argument.

  To fix this error a conditional compilation flag needs to be
  added.

- Add CXX_FLAGS += -std=c++11 to the project file. This flag is ignored
  when compiling in msvc, i guess it is harmless.