Provide global assert solution. <ac/assert.h> (new) is now included

by portable.h with NDEBUG undefined.  This makes assert() is always
available and automatically disables itself when LDAP_DEBUG is undefined.
I've included a basic assert() for pre-STDC compilers.  It relies on
abort() which may not actually be available.  (well replace abort()
with whatever is appropriate if and when we're faced with a pre-STDC
compiler that doesn't have assert()).