[bootlin/training-materials] 292167: Makefile: check for all .tex files to exist

[Clément Léger]

  Branch: refs/heads/master
  Home:   https://github.com/bootlin/training-materials
  Commit: 292167ee39305102646ad2d27bc08a80797d028e
      https://github.com/bootlin/training-materials/commit/292167ee39305102646ad2d27bc08a80797d028e
  Author: Clément Léger <clement at clement-leger.fr>
  Date:   2022-05-12 (Thu, 12 May 2022)

  Changed paths:
    M Makefile

  Log Message:
  -----------
  Makefile: check for all .tex files to exist

Missing .tex files lead to the %-slides.pdf rule not being matched and
thus displaying a message saying that there is no rule to build the
training slides. Better be cautious and checked for all files to exist.

Signed-off-by: Clément Léger <clement at clement-leger.fr>