Run autopep8 as well as clang-format when calling "make format",
the PATHS argument is passed to both utilities which will only operate
on files they support.

For example: `make format PATHS=release/scripts` formats Python scripts,
`make format PATHS=source/blender/blenlib` would format C/C++.
If users really want they can format C/C++ & Python files at the same
time since both formatting utilities filter on file extension.

`make format PATHS="release/scripts/startup/nodeitems_builtins.py source/creator/creator.c"`

A LIBDIR variable has been added to the GNUmakefile to simplify
references to this directory which can be one of 3 possible values.

Reviewed By: sybren, brecht

Ref D14789