more Unix gitinfo

more Unix gitinfo

When make uses 'makedefs -v' to create date.h, force it to create
gitinfo.txt all the time instead of just when that doesn't already
exist.  Use 'make GITINFO=0' to get the previous behavior.

To skip it entirely, you need to do that and also make sure that
some file by that name already exists.  'touch dat/gitinfo.txt' or
perhaps 'echo "#no git" > dat/gitinfo.txt' would suffice.