Fix problem with book "timeline"

From [GNU Make 3.82 release](https://lists.gnu.org/archive/html/info-gnu/2010-07/msg00023.html)

> * WARNING: Future backward-incompatibility!
>
>   Wildcards are not documented as returning sorted values, but up to and
  including this release the results have been sorted and some makefiles are
  apparently depending on that.  In the next release of GNU make, for
  performance reasons, we may remove that sorting.  If your makefiles
  require sorted results from wildcard expansions, use the $(sort ...)
  function to request it explicitly.

This PR changes `Makefile` to make it compatible with GNU Make >= 3.82.