holding the associated mutex while signaling a condition variable isn't
necessary and in some implementations will reduce performance as the
woken thread may test the mutex, fail and go back to sleep.
Change-Id: Id685a47b0c76fc4a1c5acedcb6623e8c55056415
this avoids duplicates between these trees and dsp/, e.g., enc/tree.c,
dec/tree.c, making pulling the whole library source tree into one target
possible
BUG=webp:279
Change-Id: I060a614833c7c24ddd37bf641702ae6a5eef1775
