[tex-live] configure of dvipng is broken if libgd is not installed.

[Karl Berry]

    Jan-Åke knows what I have done and he has even
    supported this

Well, so much the better.  He didn't say anything about it when I wrote
him, so I didn't know.  Anyway, I sent him our various files and
encouraged him to consider automake.  Whatever.  He clearly wants to
help, but ...

    Basically, it was not so easy to merge our autoconf stuff, because:

Yes, I know.  Not to mention kpse/ac_include.

     Hopefully, things will improve with Peter's patches (which allow us to
     use autoconf-2.5x in texk).

Indeed.