New option Syzygy50MoveRule. Set to false to let tablebase probes
ignore the 50-move rule. This is useful for correspondence chess.
Reformat Makefile

No functional change.
Further simplify Makefile

No functional change.
New option SyzygyProbeDepth.
The tables are only probed at nodes with remaining depth >= SyzygyProbeDepth.
The default value is 1, which corresponds to the old behaviour.
Set it to a higher value if probing affects nps too much.
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'syz/master' into syzygy1