> > It seems ./configure did not detect the fact that bison was missing from
> > my machine:
>
> IIRC it would try to use yacc otherwise, but your output makes me think
> you are on Linux, and I'm not sure about whether it could use yacc anyway
> if there is no bison...

No, Ruby 1.9 requires bison, yacc is not enough.

> > What? Do I need an installed ruby to build ruby from trunk or am I
> > missing something else here?
>
> I'm not familiar enough with the details of the build process to
> explain this, but I know from experience you don't need ruby to build ruby.
> I seem to recall ruby making use of itself once built to do the final bits
> of the build, and the install, but I don't really keep this stuff in my
> head.  Hoping someone else who writes this stuff can jump in here!

I am pretty sure Ruby 1.9 DOES require a working Ruby (though Ruby 1.8
does not).

Even though they are needed to compile the SVN version, bison or a
working Ruby will probably not be necessary to build the release
version.

For the fact that configure does not complain it may be for this
reason, but I am not sure. It could indeed complain if the bison or
ruby generated files are not there.