Make UnixWare 7 bootstrap support work with final shipping product.

a
        Make UnixWare 7 bootstrap support work with final shipping product.
        * configure.in: (i[34567]86-*-sysv5): append, not overwrite, xm_file.
        Pick up xm-siglist and xm-alloca.
        (xm_defines): Add USG so dbxout will build.
        * configure: Regenerate.

From-SVN: r19528