Because Boost.System has switched to define BOOST_POSIX_API on Cygwin[1],
Boost.Filesystem now defines its own set of macros for platform API
selection. At this time, we preserve the previous behavior, where
Cygwin is treated as Windows.
[1]: https://github.com/boostorg/system/pull/137
The algorithm implementation now ignores empty and dot path elements in the
argument path and accounts dot-dot elements by decreasing the number of
dot-dot elements to generate in the resulting relative path. This is
according to C++17 std::path specification [fs.path.gen]/4.
Fixes https://github.com/boostorg/filesystem/issues/76.
