Remove threading dependencies with --disable-multithread

Avoid a pthreads dependency via pthread_once() when compiled with

In addition, this synchronization is disabled for Win32 as well, even
though we can be sure that the required primatives exist, so that the
requirements on the application when built with --disable-multithread
are consistent across platforms.

Users using libvpx built with --disable-multithread in a multithreaded
context should provide their own synchronization. Updated the
documentation to vpx_codec_enc_init_ver() and vpx_codec_dec_init_ver()
to note this requirement. Moved the RTCD initialization call to match
this description, as previously it didn't happen until the first

Change-Id: Id576f6bce2758362188278d3085051c218a56d4a
7 files changed