When `rake -m` and so on are used to build in parallel, building may be
started before presym files are generated. Then, for example, the following
error occurs and this issue is fixed.

```console
In file included from /Users/shuujii/mruby/mruby/include/mruby.h:92:
/mruby/mruby/include/mruby/presym.h:16:10: fatal error: '../build/presym.inc' file not found
#include <../build/presym.inc>
         ^~~~~~~~~~~~~~~~~~~~~
```