Deprecated std::binder2nd in parallel/algo.h

[Morwenn Ed]

Since it is possible to call g++ with -D_GLIBCXX_USE_DEPRECATED=0, we should probably
get rid of the dependence of deprecated features in the rest of the library; that would
ensure the usability of _GLIBCXX_USE_DEPRECATED.

I found std::binder2nd still used in parallel/algo.h (line 153) which could probably be
replaced by __gnu_parallel::__binder2nd since the function exists and is still compiled
when _GLIBCXX_USE_DEPRECATED is set to 0.