brand new gethostbyname_r() and gethostbyaddr_r() checks that now detects

the AIX-styles as well.
I added --disable-thread to allow a user to disable all testing for
thread-safe functions being present and thus go with the good old style
functions. This might become handy if (when?) I add a bug in the threaded
code, like the AIX code I have now but can't test!