odd bootstrap fail on Solaris (3.1 branch): bad Makefile generated

[Joe Buck]

I wrote:
> I can't boot the 3.1 branch on sparc-sun-solaris2.7 without editing a
> Makefile.  The only configure flag I give is --prefix, and I try to build
> in a separate directory as usual.

I found the problem: the bison on my path was a really old one.  This is
actually detected by configure, but not in a way that anyone would be
likely to notice.  It outputs the message

checking version of bison... 1.25, bad

as the 365'th line, out of 445 lines of output, but just kept chugging along.

If the final distribution contains the output files gcc/intl/plural.c etc,
this won't be a problem.  Is that the plan?  We aren't supposed to require
bison for folks who just use the release.

As an alternative, rather than generating a colon in this case (so that
the command is a comment, it could generate a string like

sorry_your_bison_version_is_too_old

as the value of YACC.  Then if the command is not needed, everything is
fine, but if the file is out of date the user will get a hint as to what
the problem is.