This doesn't compile with gcc 8 on MinGW-w64, and fails in runtime with
gcc 10.2 and clang 8.0.1 on Cygwin64 because character code conversion errors,
so basically std::filesystem never works with wide paths on Windows.
We still use wide paths as `const wchar_t*` with libc++ though.
Also, changed BOOST_FILESYSTEM_C_STR definition to accept the path as
an argument and use that definition in the tests rather than duplicating it.
Related to https://github.com/boostorg/filesystem/issues/181.
When the source path is not absolute and copy_options::create_symlinks is
specified, deduce the relative path from the target location to the source
file to create a symlink. This allows to copy to a path that is not the current
path.
Also, added absolute overloads taking error_code argyment.
Also, when current_path() is used as a default argument to other operations,
and the operation also accepts error_code, use current_path(ec) to report
errors through the error code rather than throwing an exception.
Also, added a test for copy operation.
