Even VMS has had strdup() since at least 7.0 (1996), so stop doing
magic to reimplement it ourselves.  This is a noop on other platforms.
Stop #define'ing a bad reimplementation of strdup() for platforms that
don't have it (if indeed those listed even do still lack it).  If we
ever really have to worry about this case, we'll write up real compat
code.
Add #ifndef to avoid an unused var warning when building -DNDEBUG.
Take out compat code for pre-7.0 VMS and a few other systems that lack
strdup().  Here's a nickel, kid, go buy yourself a better computer.