Ok, when checking for old-style SSLeay headers we cannot just use

Ok, when checking for old-style SSLeay headers we cannot just use
AC_CHECK_HEADERS() and the action-if-found since that action is run even if
just one of the six headers is found and I just now fell over a case with
a duplicate file name (a krb4 implementation with an err.h file).

I converted the check to manually make sure three of the headers are present
before considering them fine.