change default PREFIX

By convention, it is strongly recommended that locally installed
software must be placed within /usr/local rather than /usr.
license under GPL version 3
only evaluate *VERSION when necessary

This avoids the following error during a `make clean`:

    fatal: No names found, cannot describe anything.
rename child variables in update_block
rename bar to status_line

This commit renames the struct bar and some related functions to
status_line, which is more relevant.