``mk_genfile_common.py`` and adapt ``mk_util.py`` and
``mk_install_tactic_cpp.py`` to use the code at its new location. The interface
has been changed slightly so that ``mk_install_tactic_cpp_internal()`` now
returns the path the generated file. The motivation behind this is so that
clients of the function know the path of the generated file.
Whilst I'm here reindent ``mk_install_tactic_cpp.py`` and the relevant
code in ``mk_util.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.
function ``mk_install_tactic_cpp_internal()`` which is called by a
new ``mk_install_tactic_cpp.py`` script. This will allow other build systems to
generate the ``install_tactic.cpp`` files.
