Fixed 'STUMPWM not found' when building

Added `STUMPWM_ASDF_DIR` in `configure.ac` to store the current directory
where stumpwm is being configured. This variable is used on both `Makefile.in`
and `make-image.lisp.in` to push the directory to `asdf:*central-registry*`
before trying to load the stumpwm package.