Will Thompson authored Apr 09, 2019 and Freedesktop SDK Merge Bot committed Apr 14, 2019

For cases where $1 has a single path component, like "man", the
calculation using `realpath` works correctly, but it's subtly wrong when
$1 is "gtk-doc/html", because the symlink path needs to be relative to
"/usr/share/gtk-doc", not "/usr/share". As a result, the symbolic link
at /usr/share/gtk-doc/html points to runtime/docs/gtk-doc/html, which
resolves to /usr/share/gtk-doc/runtime/docs/gtk-doc/html which does not
exist. The correct relative path is ../runtime/docs/gtk-doc/html, which
resolves to /usr/share/runtime/docs/gtk-doc/html which exists.

Happily, `ln -s` has a `--relative` flag which works out the right
relative path.  This likely regressed in
a8dbd2b0, before which the symlinks were
absolute.

Fixes #738.