build: Don't fail if SWIG isn't found

People from distros have reported that packaging the newer versions is
more difficult because we now depend on SWIG. We don't have to, it's
needed for the python dumper and is an optional build dependency.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/configure.ac b/configure.ac
index b71b100..6226829 100644
--- a/configure.ac
+++ b/configure.ac
@@ -132,7 +132,13 @@
 AC_ARG_ENABLE(dumper,
 	      AS_HELP_STRING([--disable-dumper],
 			     [Disable the python based register dumper (default: enabled)]),
-			     [DUMPER=$enableval], [DUMPER=yes])
+			     [DUMPER=$enableval], [DUMPER=auto])
+if test "x$DUMPER" = xauto; then
+	# AX_PYTHON_DEVEL/AX_SWIG_PYTHON are not super friendly and don't
+	# easily allow us to introspect the result of their checks. So if we
+	# find SWING on the system, that's enough to try compiling the dumper.
+	AX_PKG_SWIG(2.0.0, [DUMPER=yes], [DUMPER=no])
+fi
 if test "x$DUMPER" == xyes; then
 	AC_DEFINE(HAVE_DUMPER, 1, [Have dumper support])
 	# SWIG configuration