make -j12 (parallel job execution) causes failures

[michaelni]

I normally run make -j12 as my box has 6 cores. While build is fast enough without it should not fail.
But it did fail and running it a 2nd time succeeded but resulted in a corrupted ffv1.pdf that had 2 appendix sections
The issue is reproduceable with make clean ; make -j12

[dericed]

I suspect this is because the same tmp filename, merged.md, is used in a few places.