Detect that the environment variable MAKEFLAGS specifies a
jobserver pool to use, and automatically use it to control
build parallelism when this is the case.

This is disabled is `--dry-run` or an explicit `-j<COUNT>`
is passed on the command-line. Note that the `-l` option
used to limit dispatch based on the overall load factor
will still be in effect if used.

+ Use default member initialization for BuildConfig struct.

+ Add a new regression test suite that uses the
  misc/jobserver_pool.py script that was introduced in
  a previous commit, to verify that everything works
  properly.