This mode replaces the previous heuristic setting enabled with -O, where we
would log directory enter/leave for each synchronized output. Now we only
do that if --trace=dir is given.
If output-sync is enabled, have make write the command line to the temp file
instead of printing it directly to the screen to ensure that the output is
ordered properly. Also, remove extraneous enter/leave operations by having
them printed directly when dumping temp file output.
If we are not going to sync a command line then dump any collected output
first to preserve ordering. Do some code cleanup:
* Move the handle init to a separate function.
* Move the temp file truncation to the output function.
* Remember whether we sync in a variable for readability.
* Handle EINTR and short writes in child_out().
* Always call sync_output() in case output_sync was changed due to error.
Enhance the child_error() function so that it will write error output to the
child's sync output buffer, if it exists. If it doesn't the output goes to
stdout/stderr.
A new flag to the -O/--output-sync, "job", selects a per-job (that is, per
line of a recipe) output synchronization. To support this move the close of
the temp file out of the sync_output() function and don't do it until we free
the child, since we may call sync_output() multiple times in a given recipe.
When we set up for a new temp file, if we're in per-job mode we truncate the
file and seek to the beginning to re-use it for every job.
