patch gnutls to not try to use pthread_atfork, which is not available on android

It has a configure test for pthread_atfork, but seems to fail to do
anything with it. I also cannot find any place where the _gnutls_atfork
function is actually used. Altogether weird..
avoid memxor conflict

both nettle and gsasl define memxor, which causes a link failure
when linking them statically into the same binary