Improve error message for _WIN32_WINNT < 0x0600.

Tom Finegan suggested the new error message.

Change-Id: I8b90f03314dadae8ee17154291a1de8de5b24e91
diff --git a/aom_util/aom_thread.h b/aom_util/aom_thread.h
index 071e15e..8d04312 100644
--- a/aom_util/aom_thread.h
+++ b/aom_util/aom_thread.h
@@ -35,7 +35,7 @@
 typedef CRITICAL_SECTION pthread_mutex_t;
 
 #if _WIN32_WINNT < 0x0600
-#error Windows Vista / Server 2008 or greater is required.
+#error _WIN32_WINNT must target Windows Vista / Server 2008 or newer.
 #endif
 typedef CONDITION_VARIABLE pthread_cond_t;