configure fails if GnuTLS not present

This patch will allow it to complete, so that only the MSN module 
doesn't build, rather than all of eb.

Also, why not use openssl?  Ayttm currently works with openssl instead 
of gnutls - we could pick whichever exists.

Philip

Index: configure.ac
===================================================================
--- configure.ac	(revision 269)
+++ configure.ac	(working copy)
@@ -191,7 +191,7 @@
         fi
     done
     if test x_$found_tls != x_yes; then
-        AC_MSG_ERROR(Cannot find GnuTLS libraries)
+        AC_MSG_WARN(Cannot find GnuTLS libraries)
     else
         AC_MSG_RESULT($tlsdir)
 if test "$use_mingw" != 'no'; then