build: Use missing' for dot'.

* configure.ac: Substitute `DOT'.
* doc.am (.dot.png, .dot.eps): Use $(DOT).

configure.ac

@@ -121,6 +121,10 @@ AC_CACHE_SAVE
 
 m4_include([config-daemon.ac])
 
+dnl `dot' (from the Graphviz package) is only needed for maintainers.
+dnl See `HACKING' for more info.
+AM_MISSING_PROG([DOT], [dot])
+
 AC_CONFIG_FILES([Makefile
                  po/Makefile.in
 		 guix/config.scm])

doc.am

@@ -33,11 +33,11 @@ DOT_OPTIONS =						\
   -Nfontsize=9 -Nheight=.1 -Nwidth=.1
 
 .dot.png:
-	dot -Tpng $(DOT_OPTIONS) < "$<" > "$@.tmp"
+	$(DOT) -Tpng $(DOT_OPTIONS) < "$<" > "$@.tmp"
 	mv "$@.tmp" "$@"
 
 .dot.eps:
-	dot -Teps $(DOT_OPTIONS) < "$<" > "$@.tmp"
+	$(DOT) -Teps $(DOT_OPTIONS) < "$<" > "$@.tmp"
 	mv "$@.tmp" "$@"
 
 # We cannot add new dependencies to `doc/guix.pdf' & co. (info "(automake)