don't build mod_ilbc or mod_siren by default

Our default build probably shouldn't include non-free software.  With
mod_ilbc, the licensing situation is merely ambiguous.  With
mod_siren, the user can't use this code without getting explicit
permission from Polycom (though it is apparently easily given).

build/modules.conf.in

@@ -58,12 +58,12 @@ codecs/mod_bv
 codecs/mod_g723_1
 codecs/mod_g729
 codecs/mod_h26x
-codecs/mod_ilbc
+#codecs/mod_ilbc
 #codecs/mod_isac
 #codecs/mod_opus
 #codecs/mod_sangoma_codec
 #codecs/mod_silk
-codecs/mod_siren
+#codecs/mod_siren
 codecs/mod_speex
 dialplans/mod_dialplan_asterisk
 #dialplans/mod_dialplan_directory