``mk_genfile_common.py``. A new function ``mk_hpp_from_pyg()`` has been added
which provides a more sensible interface (hides the nasty ``exec()`` stuff)
to create the ``*.hpp`` files from ``*.pyg`` files.
Both ``mk_util.py`` and ``pyg2hpp.py`` have been modified to use the new
interface.
Whilst I'm here reindent ``pyg2hpp.py``.
The purpose of this change is to have Python code common to the Python
and CMake build systems separate from Python code that is only used for
the Python build system.
I've tested this change by making sure that the all the ``*.hpp``
files generated from ``*.pyg`` files match the files generated
before this change.
