Dispatch dl_expandspec at build time, not run time.

Dispatch dl_expandspec at build time, not run time.

Before 0a0b6c96e6042658aa123b3990724be593a2bb63, dl_expandspec was
an autoloaded function that would never be called if an XS version
was already loaded.  Without autoloading in DynaLoader anymore,
the pure Perl version gets loaded first, which causes trouble if
the XS version is what's expected.  It looks as though VMS is the
only platform that has an XS version, so just avoid declaring the
Perl version there.