gimpdir goes to ~/Library/Gimp/x.y
thumbnails go to ~/Library/Caches/org.freedesktop.thumbnails

The thumbnail location is not standardized yet, but is the only
location that makes sense. Also fix user install to search old
gimpdirs to migrate in both Library and the classic location.
Remove the obsolete CABON_CFLAGS from all makefiles.
(cherry picked from commit 9112d684)