Instead of simply warning, produce an error if gob2 is requested but
not present. Patch taken from
https://bugs.launchpad.net/ubuntu/+source/gob2/+bug/317801

--- gob2-2.0.17.orig/gob2.m4
+++ gob2-2.0.17/gob2.m4
@@ -54,5 +54,5 @@
 ])
 
 AC_DEFUN([GOB2_CHECK],[
-	GOB2_HOOK($1,[],[AC_MSG_WARN([Cannot find GOB-2, check http://www.5z.com/jirka/gob.html])])
+	GOB2_HOOK($1,[],[AC_MSG_ERROR([Cannot find GOB-2, check http://www.5z.com/jirka/gob.html])])
 ])