Look for boost libraries installed on the Mac by port, brew, or fink.

As mentioned in #435, build fails on the Mac unless BOOSTDIR is set to the correct directory.

I have modified the setenv_mac*.sh scripts to set the environment variable based on what libraries it finds.  (i.e. Fix 2 of the problem report).  I have only tested it on my system which had Boost (multi-threaded) installed by macports.

tests/FindBoost.cmake might benefit form having /opt/local added to the list of of search paths.  however, that is not a complete solution because it doesn't allow boost.pri to make a distinction between single- and multi-threaded versions of the library.  I don't know enough about the various make systems in use to find a good solution.

I also modified the README.md to refer to the correct setenv*.sh names.