We now use a script named gen_id to generate the base build_id/host_id.
This solves 2 problems:
1. GNU Make special-casing exit code 127 (command not found) meant that
warnings about missing tools would propagate to the user's terminal
and broke our opportunistic build_id construction.
2. This change ensures that we don't have arbitrary characters in our
make variables that would be misinterpreted by Make.
See comments in depends/Makefile and depends/gen_id for more
information.
