Rather than using pre-generated files, the *.xml files describing the
Why3 theories to be realized are generated again before compiling the
corresponding Isabelle theories. Instead of the generated *.xml files,
we use a file containing their hash values to detect changes in the
realizations. Since there may be different realizations for different
versions of Isabelle, we provide a file with hash values for every
supported version of Isabelle. The files containing the hash values
can be updated via the update-isabelle target.

(cherry picked from commit 039e0f0a321c36ea3bea231e4376f5833cd2ad8a)