this provides stronger synchronization when pthreads are available as
was done in 'd77bf512 add WEBP_DSP_INIT / WEBP_DSP_INIT_FUNC' for the
other init functions.
Change-Id: I2ffe4e24454d276c2411ece34dca38d23d4756d5
this internalizes the init checks and provides stronger synchronization
with pthreads when available while still allowing VP8GetCPUInfo to be
modified (mostly for testing purposes). windows is left as is since a
critical section or mutex would cause a leak.
Change-Id: Ieb997e014f2805c0ae39c16f13337663521356f4
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