``mk_genfile_common.py`` and adapt ``mk_util.py`` and
``mk_def_file.py`` to use the code at its new location.
Whilst I'm here also reindent ``mk_def_file.py`` and make it
use some of the code in ``mk_genfile_common.py`` to avoid code
duplication.
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.
function ``mk_def_file_internal()`` which is called by a
new ``mk_def_file.py`` script. This will allow other build systems to
generate the ``api_dll.def`` file.
