0.8.15.18: Linkage table tweaks & alien bugfix

0.8.15.18: Linkage table tweaks & alien bugfix
            * Build with linkage-table by default on x86/NetBSD and
               sparc/Linux as well.
            * Don't try to be too clever about when to warn user about
               alien definitions when saving cores on non-linkage-table
               platforms: do it unconditionally.
            * Fix parsing of recursive alien record and union types
               (reported by Thomas F. Burdick, port of Helmut Eller's
               patch for the same problem in CMUCL.)