build: Fix build configuration to properly support split build dir

Building LXC in a separate target directory, by running configure from
outside the source tree, failed with multiple errors, mostly in the
Python and Lua extensions, due to assuming the source dir and build dir
are the same in a few places. To fix that:

- Pre-process setup.py with the appropriate directories at configure
  time
- Introduce the build dir as an include path in the Lua Makefile
- Link the default container configuration file from the alternatives
  in the configure stage, instead of setting a variable and using it
  in the Makefile

Signed-off-by: Daniel Miranda <danielkza2@gmail.com>