2.7.51 released March 18, 2000

- doc revision
- hurry >= 3 turns off some features
- adjust configure.in, config.vc and sgf/ttsgf.c
  to test for vsnprintf instead of snprintf
- replace an embedded newline in a #defined
  string in main.c with an explicit \n
- remove trailing ',' characters in enum
  declarations
- include interface.h in sgf_utils.h so
  that enum testmode is defined when declaring
  guess_mode_from_sgf_comment.
- bugfix in break_through_helper()