wscript: mark readline as optional

We need readline for transport.c, but it's built conditionally if
present, so allow the check for readline to fail.

example-clients/wscript

@@ -44,7 +44,7 @@ def configure(conf):
 
     conf.check_cfg(package='celt', atleast_version='0.5.0', args='--cflags --libs', mandatory=False)
 
-    e = conf.check_cc(lib='readline', define_name="HAVE_READLINE")
+    e = conf.check_cc(lib='readline', define_name="HAVE_READLINE", mandatory=False)
 
     # define_name="HAVE_READLINE" has no effect, LIB_READLINE is defined if readline is available
     #if conf.is_defined('HAVE_READLINE'):