swig: search for swig4.0

* configure.ac: Use swig4.0 when available.
* HACKING: Update.

HACKING

@@ -32,7 +32,9 @@ since the generated files they produce are distributed.)
     a separate and more recent org-mode package, or you can
     simply upgrade from ELPA).
   Groff (a.k.a. GNU troff) >= 1.20
-  SWIG >= 3.0 (for its better C++11/C++14 support)
+  SWIG >= 3.0.2 (for its better C++11/C++14 support)
+    Note that if you use Macports you should consider to install the
+    swig-python package in addition to the swig package.
   Doxygen >= 1.4.0
   R   used by some examples in the documentation
   Perl, with its Gettext module (it might be called something like

configure.ac

@@ -195,9 +195,10 @@ AC_CHECK_PROG([EMACS], [emacs], [emacs])
 AC_CHECK_PROGS([IPYTHON], [ipython3 ipython], [ipython])
 AC_CHECK_PROG([LBTT_TRANSLATE], [lbtt-translate], [lbtt-translate])
 AX_CHECK_VALGRIND
-# Debian has a binary for SWIG 3.0 named swig3.0 and they kept swig as
-# an alias for swig-2.0.  Let's use the former when available.
-AC_CHECK_PROGS([SWIG], [swig3.0 swig], [swig])
+# Debian used to reserve the name 'swig' for swig-2.0.  So prefer
+# swig4.0 (available in Debian bullseye) to swig3.0 (available in Debian buster)
+# ti swig.
+AC_CHECK_PROGS([SWIG], [swig4.0 swig3.0 swig], [swig])
 
 AC_SUBST([CROSS_COMPILING], [$cross_compiling])