Use CMake to build project

I suggest to use CMake to build project. It supports SDL and OpenGL.

#### Summary of commits

* Prepare to use cmake (>= 3.0) to build `duck` and `gduck`
* Add CMakeLists.txt
  * e.g. https://github.com/sh19910711/duck-lang/blob/ad8b6fe83b1145c41f7724980d5e6c88ff94d0e7/src/CMakeLists.txt
* Add source directory (in src/)
* Add FLAG_GDUCK switch
  * e.g. `$ gcc ... -DFLAG_GDUCK -o gduck ...`

```shell
$ git clone https://github.com/gregtour/duck-lang
$ cd duck-lang
$ cmake .
$ make
=> generates bin/duck and bin/gduck
```

I have tested the commands only on linux, and I don't touch files and directories without source directory.