[boost] Solaris versions of sp_counted_base and atomic_count by Michael van der Westhuizen

Michael van der Westhuizen has generously contributed Solaris versions of atomic_count and sp_counted_base. You can find them in the CVS HEAD and as attachments to this message. I haven't applied his patch (also attached) to atomic_count.hpp and sp_counted_base.hpp that would enable them yet, but I plan do so in a couple of days if there are no objections. If you have any suggestions or comments, please let me know; in particular, if you have a better idea how to detect SunOS 5.10 or later, or if you know what memory synchronization guarantees the Solaris atomics are supposed to provide. :-) Thanks in advance for any help or interest, and of course, big thanks to Michael for his contribution.