parallel make fixes the 3rd

parallel building on (at least) linux still fails, even after ad815563a3837bbd9ea73c3645af55ec56cacb9a
So its time for a hammer: use locking

This has has survived several manual compiles by different users as well as a launchpad build for 4 ubuntu series.
The downside is that it can slow down ffmpeg building to, worst case, single threaded

@T-nelson: this disables the ffmpeg binary again