MinGW: use off64_t and lseek64()

On Windows, off_t is restricted to 32-bit, even if it is perfectly
capable of using 64-bit offsets; Force the use of 64-bit offsets
by overriding the declarations of off_t and lseek() with their 64-bit
counterparts.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.h

@@ -117,6 +117,9 @@ static inline int waitpid(pid_t pid, int *status, unsigned options)
 	return -1;
 }
 
+#define off_t off64_t
+#define lseek lseek64
+
 /*
  * implementations of missing functions
  */