MinGW: Fix stat definitions to work with MinGW runtime version 4.0

For an overview of changes in mingwrt-4.0 see:

    http://sourceforge.net/p/mingw/mingw-org-wsl/ci/4.0.0/tree/NEWS

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -271,11 +271,20 @@ static inline int getrlimit(int resource, struct rlimit *rlp)
 #define lseek _lseeki64
 
 /* use struct stat with 64 bit st_size */
+#ifdef stat
+#undef stat
+#endif
 #define stat _stati64
 int mingw_lstat(const char *file_name, struct stat *buf);
 int mingw_stat(const char *file_name, struct stat *buf);
 int mingw_fstat(int fd, struct stat *buf);
+#ifdef fstat
+#undef fstat
+#endif
 #define fstat mingw_fstat
+#ifdef lstat
+#undef lstat
+#endif
 #define lstat mingw_lstat
 
 #ifndef _stati64