0.9.18.71: fix build on Darwin 7.9.0 (OS X 10.3)

 * Unix98 pty api not supported here: don't add the ldso-stubs for it.
   (Not needed in the presence of linkage-table anyways. After 1.0 the
   conditionalization should be changed to :linkage-table.)

 * ...which still leaves us with the alien definitions. They are
   harmless even if the Unix98 api is not supported, as the code path
   where they live is executed only if we can open /dev/ptmx. They
   will, howver, cause undefined alien style-warnings on startup.

   Since we presumably want to try to keep the same core working on
   both older and newer Darwins, we must detect its support at
   runtime.

   So just muffle undefined alien style warnings from the linkage
   table at startup -- presumably user-code can face similar
   situations, so this is probably TRT anyways.

 * Whitespace damage.

 This commit message is longer then the patch.