Small XDG_DATA_DIRS improvements

- win32: improve iss script to only replace the real XDG_DATA_DIRS path, not the comment
- use $(datadir) instead of $(prefix)/share, for the rare case someone configures with
  --datadir=DIR

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23337 57a11ea4-9604-0410-9ed3-97b8803252fd