Add "3" suffix to default user config file

Index: configure
--- configure.orig
+++ configure
@@ -2453,7 +2453,7 @@ if test "$sysconfdir" = '${prefix}/etc'; then
     system_xpdfrc="$prefix/etc/xpdfrc"
   fi
 else
-  system_xpdfrc="$sysconfdir/xpdfrc"
+  system_xpdfrc="$sysconfdir/xpdfrc3"
 fi
 cat >>confdefs.h <<_ACEOF
 #define SYSTEM_XPDFRC "$system_xpdfrc"
