There's no need anymore to call an own error handler directly. Instead we
register our error handler and call libgd's error functions (which will
forward). We do this regardless of compiling with the bundled or an external
libgd.
This initial checkin has no changes to any of the libgd code so it can
be used as a basis for diffs. It also will not build currently because
of this. The PHP gd checks need to be incorporated along with a bit of
other config magic. It also shouldn't break the build and will only
take effect if you use --with-gd=php right now.
