thread: use native windows cond var if available
thread: use native windows cond var if available

Vista / Server 2008 and up. no speed difference observed.

Change-Id: Ice19704777cb679b290dc107a751a0f36dd0c0a9

PKRoma/libwebp,ttyangf/libwebp,webmproject/libwebp,ttyangf/libwebp,redengineer/libwebp,redengineer/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,redengineer/libwebp,redengineer/libwebp,ttyangf/libwebp,webmproject/libwebp,PKRoma/libwebp,ttyangf/libwebp,PKRoma/libwebp,ttyangf/libwebp,redengineer/libwebp