Modernize code

Can compile with arm-none-eabi-gcc 8.3.0
gbafix correctly handles ELF input

src/save.c

@@ -626,7 +626,10 @@ static u16 CalculateChecksum(void *data, u16 size)
     u32 checksum = 0;
 
     for (i = 0; i < (size / 4); i++)
-        checksum += *((u32 *)data)++;
+    {
+        checksum += *((u32 *)data);
+        data += sizeof(u32);
+    }
 
     return ((checksum >> 16) + checksum);
 }