Depend on image files in Makefile

For this to work, all images included in tex files must be given with their file
suffix.  For this reason, I also had to update some of these text files.

Makefile

@@ -42,13 +42,13 @@ $(foreach presentation, $(PRESENTATIONS), \
   $(eval $(call PRESENTATION_template, $(presentation))))
 
 # This template is called with a single argument like
-# ‘presentations/content/ccc/ccc_bundesweit.tex’; it generates a rule that does
-# nothing so far.  In the future, it could depende on all images used by the tex
-# file.
+# ‘presentations/content/ccc/ccc_bundesweit.tex’; it generates a rule that
+# dependes on all images used by the tex file.  The actual recipe just checks
+# whether the tex file is existent and updates the timestamp.
 
 define DECK_template
-$(1):
-	:
+$(1): $(shell perl -ne '/includegraphics[^\{]*\{([^\}]*)\}/ && print "presentations/", $$1, " "' $(1))
+	test -f $(1) && touch $(1)
 endef
 
 # Files contained in decks are only considered in the top-level directory

presentations/content/alternate/alternate.tex

@@ -94,7 +94,7 @@ Plattformübergreifende Synchronisierung von Dateien, Dokumenten, Kalendern, Kon
       \includegraphics{img/fdroid.png}
       \column{5cm}
       \begin{center}
-        \includegraphics[width=2cm]{img/F-Droid_Logo_2}
+        \includegraphics[width=2cm]{img/F-Droid_Logo_2.pdf}
         \par\end{center}
         \begin{center}
           \par

presentations/content/geraetesicherheit/foss_programme_apps.tex

@@ -39,7 +39,7 @@
     \column{5cm}
 
     \begin{center}
-      \includegraphics[width=2cm]{img/F-Droid_Logo_2}
+      \includegraphics[width=2cm]{img/F-Droid_Logo_2.pdf}
     \par\end{center}
     \begin{center}
     \par\end{center}

presentations/content/geraetesicherheit/unerwuenschte_funktionalitaet.tex

@@ -10,7 +10,7 @@
 \begin{frame}
   \frametitle{Unerwünschte Funktionalität}
   \begin{center}
-    \includegraphics[width=7cm]{img/backdoor-apps}
+    \includegraphics[width=7cm]{img/backdoor-apps.png}
   \par\end{center}
 \end{frame}
 

presentations/content/motivation/freesoftware.tex

@@ -31,11 +31,11 @@
     \column{7cm}
 
     \begin{center}
-      \includegraphics[width=4.5cm]{img/stallman}
+      \includegraphics[width=4.5cm]{img/stallman.jpg}
     \par\end{center}
 
     \begin{center}
-      \includegraphics[width=5cm]{img/logo-fsf}
+      \includegraphics[width=5cm]{img/logo-fsf.pdf}
     \par\end{center}
   \end{columns}
 \end{frame}
@@ -79,7 +79,7 @@
     \column{5cm}
 
     \begin{center}
-      \includegraphics[width=2cm]{img/F-Droid_Logo_2}
+      \includegraphics[width=2cm]{img/F-Droid_Logo_2.pdf}
     \par\end{center}
     \begin{center}
     \par\end{center}