[boost] [btree] Library working with more compilers and standard libraries

* VC++ 10 and MinGW/gcc 4.5 are my primary platforms, and the tests continue to pass on those. Some of the tests have been beefed up, too. * The Btree library now passes tests with Cygwin/gcc 4.3. That was a bug in my POSIX implementation code that is now fixed. * VC++ 9 is working OK in release mode. There is a bug in some debug mode code in the standard library <algorithm> header's lower_bound and upper_bound functions. I'm not sure how to deal with that. Maybe supply lower_bound/upper_bound implementations, and switch to them when needed. * I'm about to start a round of tests on Linux. --Beman