Fix compilation on OS/2

Fix compilation on OS/2

_beginthread() is not declared on __STRICT_ANSI__ mode.

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    [CXX] test/quantize_test.cc.o
In file included from ./vp8/common/threading.h:194:0,
                 from ./vp8/encoder/onyx_int.h:24,
                 from test/quantize_test.cc:24:
./vpx_util/vpx_thread.h: In function 'int pthread_create(TID*, const void*, void* (*)(void*), void*)':
./vpx_util/vpx_thread.h:259:20: error: '_beginthread' was not declared in this scope
   tid = (pthread_t)_beginthread(thread_start, NULL, 1024 * 1024, targ);
                    ^~~~~~~~~~~~
./vpx_util/vpx_thread.h:259:20: note: suggested alternative: 'thread'
   tid = (pthread_t)_beginthread(thread_start, NULL, 1024 * 1024, targ);
                    ^~~~~~~~~~~~
                    thread
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Change-Id: I774a071162b3876a7f3253ce7c5749f1b0b45818