Sorry for the confusion, Richard.
Since pg requires bison 1.875 or greater, bison should be installed if the current installed version of bison is _less_than_ 1.875
As I understand the two part question number 1 above, the answer to the first part is:
No, it should not be 'smaller than or equal to'
Regarding the second part, I'm not sure why it's required. You can probably find out by googling at postgresql. Since bison parses text, it might have to do with how psql works or something.
