GCALC_DBUG_OFF and related infrastructure defined so we can enable/disable debugging conveniently.

per-file comments:
  sql/gcalc_slicescan.cc
GIS library code cleanup.
  sql/gcalc_slicescan.h
GIS library code cleanup.
  sql/gcalc_tools.cc
GIS library code cleanup.
  sql/gcalc_tools.h
GIS library code cleanup.