fixed some warnings and compilation under mingw :

- replaced _MSC_VER by _WIN32 define
- removed search.h include (not available here and tsearch() doesn't seem to be used anyway)

lua/lsave.cpp

@@ -1,6 +1,5 @@
 #include <stdlib.h>
 #include <stdio.h>
-#include <search.h>
 #include <assert.h>
 
 #include "ltask.h"