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.
NOTE: On Posix, the pipe-based protocol is not supported and
will be detected. Ninja will print a warning on startup then
ignore the content of MAKEFLAGS (there is a regression test
for this).
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.
