non-POSIX use in yacc with getdate.y

[Osipov, Michael]

Hi folks,

I have recently compiled MIT Kerberos 1.16.1 on HP-UX and yacc failed with:

> "./kadmin/cli/getdate.y", line 180: fatal error: invalid escape, or illegal reserved word: expect

Change introduced in 
https://github.com/krb5/krb5/commit/28fd0a934cdc7b3b42ce213c6d334d4edf1ab591#diff-db7f4e6f3883dd26b97f7b3e10697846R180.

I contacted HPE about this and it turned out that their yacc 
implementation does not support extensions as the GNU or BSD 
counterparts do.

I'd like to have this fixed somehow and there are several ways to do this:

* Remove this line manually before compiling on the platform which does 
not support this "%expect" extension
* Revert the commit and live with the verbose output
* Test for this extension in the configure.ac script
* Require GNU bison if yacc is POSIX only. Note I am reluctant to this 
because this is merely one line for less verbose output?

Any comments from the devs?

Regards,

Michael