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

[Thomas Esser]

> again.  (Apparently all the support which Thomas wrote has been ignored
> or they've never seen them.  Sigh.)

Both is not true. Jan-Åke knows what I have done and he has even
supported this in the sense that if he had changed something, he has
given advice for me to follow this in autoconf-2.13.

Basically, it was not so easy to merge our autoconf stuff, because:
  - my stuff supports uninstalled libraries (kpathsea, libpng etc),
    making assumptions about certain features (standard autoconf checks
    don't work for uninstalled stuff)
  - my stuff was based on autoconf-2.13, his on autoconf-2.5x

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

Thomas