Turn the error on anonymous structs/unions into a warning.

Otherwise we get too many errors on glibc's standard headers.
A real error will occur when the anonymous struct/union is accessed.

Author: xavierleroy

cparser/Elab.ml

@@ -640,7 +640,7 @@ and elab_field_group env (Field_group (spec, fieldlist, loc)) =
     error loc "non-default storage in struct or union";
   if fieldlist = [] then
     if is_anonymous_composite spec then
-       error loc "ISO C99 does not support anonymous structs/unions"
+       warning loc "ISO C99 does not support anonymous structs/unions"
     else
        warning loc "declaration does not declare any members";