use wget or curl according to what is available

* doc/Makefile.am, elisp/Makefile.am: here.
* HACKING: add missing requirements.

HACKING

@@ -38,6 +38,7 @@ since the generated files they produce are distributed.)
   Python >= 3.3, IPython >= 2.3
   GraphViz
   Java >= 1.7 (needed to run PlantUML while generating the doc)
+  wget or curl (needed to download PlantUML)
 
 The following additional tools are used if they are present, or
 only for certain operations (like releases):

doc/Makefile.am

@@ -62,7 +62,7 @@ org:
 	$(EMACS) --batch -Q -l org/init.el
 
 org/plantuml.jar:
-	wget http://downloads.sourceforge.net/project/plantuml/plantuml.jar -O org/plantuml.jar
+	wget http://downloads.sourceforge.net/project/plantuml/plantuml.jar -O org/plantuml.jar || curl http://downloads.sourceforge.net/project/plantuml/plantuml.jar -o org/plantuml.jar
 
 org-man:
 	mkdir -p $(srcdir)/userdoc/man

elisp/Makefile.am

 ## -*- coding: utf-8 -*-
-## Copyright (C) 2015, 2016 Laboratoire de Recherche et Développement
+## Copyright (C) 2015, 2016, 2017 Laboratoire de Recherche et Développement
 ## de l'Epita (LRDE).
 ##
 ## This file is part of Spot, a model checking library.
@@ -23,5 +23,5 @@ GIT = https://gitlab.lrde.epita.fr/spot/emacs-modes/raw/master/
 
 .PHONY: update-el
 update-el:
-	wget $(GIT)/hoa-mode.el -O $(srcdir)/hoa-mode.el
+	wget $(GIT)/hoa-mode.el -O $(srcdir)/hoa-mode.el || curl $(GIT)/hoa-mode.el -o $(srcdir)/hoa-mode.el
 	chmod a-w $(srcdir)/hoa-mode.el