Bison 1.28c intl/plural.c is out of date

[Paul Eggert]

The file intl/plural.c distributed with Bison 1.28c is out of date: it
was generated by Bison 1.28, not Bison 1.28c.

There is a bit of a chicken-and-egg problem here, but perhaps Bison's
build procedure should be changed so that it automatically checks,
before making a distribution, that the Bison version number in
intl/plural.c matches the version number of the Bison distribution.