Compatible Coq 8.9

Changes between this release and previous ones ensure that the code compile correctly with coq-8.9

Extra release file named coq-dpdgraph-0.6.4.tgz can robustly be compiled on most architectures. This file is also the reference for opam package management.

Default release files v0.6.4beta.zip and v0.6.4beta.tar.gz can be compiled only on architecture that provide the autoconf tool.

v0.6.3beta

corrects a mistake in the compilation instructions

Version for use in opam

We prefer to have versions for use in opam that do not depend on autoconf
This is the only difference between this version and version tagged coq-dpdgraph-0.6.2

Normally, I would not produce the previous tagged version, but there was a mishap this time.