when building courier, openssl libraries are preferred over gnutls when both are present. The configure script accepts option "--with-gnutls" that will prefer gnutls in such case. Please add "gnutls" as USE flag. It should depend on gnutls in such case, of course.
Created attachment 207974 [details, diff] patch to use gnutls with courier
*** Bug 373995 has been marked as a duplicate of this bug. ***
This is already the case, seems it was just forgotten to close this bug.