sn-4.5.1 on sco 3.2v5.0.5

[Taj Khattra]

some issues when compiling sn-4.5.1 on sco 3.2v5.0.5

- db/btree/bt_open.c: MAXPATHLEN is not #defined on sco.
  however, a missing MAXPATHLEN seems to be handled in
  other places in the code.

- 'make install' seems to depend on gnu tar  (at least sco's
  tar failed to handle some of the options) ?

- i think the 'install-data-local' target in
  snavigator/doc/html/Makefile needs a 'cd ../..' line at the
  end of the for-loop, else the second directory install
  (progref) fails.

-taj