meta-output.mk diff output

We construct output such that it should match what we
can extract from output.meta, so diff the two.
This way the test cannot pass if meta.c is capturing
something inaccurately - other than the trailing newline.

meta-output.mk: Allow for echo that does not support -n

On latest Darwin, /bin/sh builtin echo does not do -n

There will be others, so test if echo -n works and adjust.

Capture only the command output from the meta file so
that results can be consistent whether echo -n works or not.

make: ensure output captured in meta file is accurate

In CollectOutput() a newline before the end of job->outBuf
results in a short write to stdout, but meta_job_output()
was recording the entire string in the meta file, so on
next call the left overs would be duplicated in the meta file.

Pass a length to meta_job_output() if it should only report part of
the data in meta file.