- define new type of pkg dependency rule ("=pkg_x"), less strict than   the "+" rule, that add the dependent pkg (pkg_x) unless it's disabled   (-pkg_x in packages.conf);
- define new type of pkg dependency rule ("=pkg_x"), less strict than
  the "+" rule, that add the dependent pkg (pkg_x) unless it's disabled
  (-pkg_x in packages.conf);

altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h