atomic_count_gcc.hpp does not compile on cygwin gcc 3.3.3

[forester22]

this 
    operator long() const
    {
        return __exchange_and_add(&value_, 0);
    }

causes error because on cygwin __exchange_and_add() is
declared as:

static inline _Atomic_word 
__attribute__ ((__unused__))
__exchange_and_add (volatile _Atomic_word *__mem, int
__val);

value_ must be made mutable or cast must be added in
operator long() const.