make FSEEK macro for using _fseeki64 instead of fseeko on windows

vm/io.c

@@ -179,7 +179,7 @@ void primitive_fseek(void)
 		break;
 	}
 
-	if(fseeko(file,offset,whence) == -1)
+	if(FSEEK(file,offset,whence) == -1)
 	{
 		io_error();
 

vm/os-genunix.h

@@ -1,5 +1,6 @@
 #define DLLEXPORT
 #define NULL_DLL NULL
+#define FSEEK fseeko
 
 void c_to_factor_toplevel(CELL quot);
 void init_signals(void);

vm/os-windows.h

@@ -20,6 +20,7 @@ typedef wchar_t F_CHAR;
 #define STRNCMP wcsncmp
 #define STRDUP _wcsdup
 #define MIN(a,b) ((a)>(b)?(b):(a))
+#define FSEEK _fseeki64
 
 #ifdef WIN64
 	#define CELL_FORMAT "%Iu"