vpx_once: implement once() for OS/2

Change-Id: I9f736f299490464bbdbb6cd24ee6f5b46ad45ec6

vpx_ports/vpx_once.h

diff --git a/vpx_ports/vpx_once.h b/vpx_ports/vpx_once.h
index 182892a..bd9eebd 100644
--- a/vpx_ports/vpx_once.h
+++ b/vpx_ports/vpx_once.h
@@ -73,6 +73,33 @@
 }
 
 
+#elif CONFIG_MULTITHREAD && defined(__OS2__)
+#define INCL_DOS
+#include <os2.h>
+static void once(void (*func)(void))
+{
+    static int done;
+
+    /* If the initialization is complete, return early. */
+    if(done)
+        return;
+
+    /* Causes all other threads in the process to block themselves
+     * and give up their time slice.
+     */
+    DosEnterCritSec();
+
+    if (!done)
+    {
+        func();
+        done = 1;
+    }
+
+    /* Restores normal thread dispatching for the current process. */
+    DosExitCritSec();
+}
+
+
 #elif CONFIG_MULTITHREAD && HAVE_PTHREAD_H
 #include <pthread.h>
 static void once(void (*func)(void))