Documentation on intra mode prob modelling
Include stdio.h for strerror().

Fixes a warning on clang.
More pvq_encoder/pvq_decoder tests, including some failing ones.
Make tools detect OpenMP at compile time so they build with clang.

You wouldn't actually want to use the tools built this way, but this
 makes it possible to run the clang static analysis and undefined
 behavior diagnostic tools on them. It also works around the fact
 that a failure to build the tools breaks the entire build and makes
 the codebase as a whole unbuildable for clang users.
Support level 3 motion vectors.
Level 4 wip.