POSIX_BUILTINS incorrectly prohibits exporting a readonly variable. All 
other POSIX shells allow this and there is nothing in the POSIX text[*] 
that says it's not allowed. The attached patch fixes this.

Thanks,

- M.

[*] 
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_22