CXX-349 make it possible to specify dylib path on non-windows

--dllpath option has been renamed to --runtime-library-search path and made to work on non-windows. Values are now comma separated for consistency with other options.
Any reason not to use AppendENVPath for these other cases? I think it should do the right thing.

You could just use the same loop but change the variable from 'PATH' to the appopriate one.