Cleaner support for usleep() (with the correct header :)

Originally committed as revision 1669 to svn://svn.ffmpeg.org/ffmpeg/trunk

os_support.h

+#ifndef _OS_SUPPORT_H
+#define _OS_SUPPORT_H
+
+/*
+ * miscellaneous OS support macros
+ *
+ * For now:
+ * usleep()
+ */
+
+#ifdef __MINGW32__
+#  include <windows.h>
+#  define usleep(t)    Sleep((t) / 1000)
+#endif
+
+#ifdef __BEOS__
+#  ifndef usleep
+#    include <OS.h>
+#    define usleep(t)  snooze((bigtime_t)(t))
+#  endif
+#endif
+
+#if defined(CONFIG_OS2)
+#  include <stdlib.h>
+#  define usleep(t)    _sleep2((t) / 1000)
+#endif
+
+#endif /* _OS_SUPPORT_H */