Deal correctly with wx directory being a symlink in docs generation script.

Deal correctly with wx directory being a symlink in docs generation script.

The script calling Doxygen is smart enough to strip the path of the wxWidgets
directory from the paths of the files, but it didn't do it correctly if this
directory was a symlink.

Fix this by using readlink, if available, to get the canonical name.

Closes #14796.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72839 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775