relative(path a_child, path a_base) fails if a_base contains symlinks on Windows

[IncineratorBurns]

relative() calls weakly_canonical() passing a_base as argument, which in turn calls canonical(), which returns an empty path, and then relative() throws.
The message reads:
boost::filesystem::relative: The system cannot find the file specified: "T:\src\wxWidgets\interface\wx"

This wxWidgets is a symlink to wxWidgets-3.1.0.
If instead of "T:\src\wxWidgets\interface\wx" I call relative() with "T:\src\wxWidgets-3.1.0\interface\wx", the function succeeds.

[Lastique]

Probably fixed by #100.