This removes the deprecated malloc.h header Autoconf check on *nix
systems and its HAVE_MALLOC_H symbol. It can be replaced mostly with the
stdlib.h. The libgd usptream also doesn't include it anymore.
On Windows, it is still used for some memory allocation functions, but
can be replaced with stdlib.h in the future.
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.
