Remove autotools dependency

This is a evaluation-only pull request, not to be merged (see it as a request for comment).

I only added a makefile into `src/`, which can be used to compile toxic without executing the `configure` script.
This also fix Tox/toxic#143 (since `_XOPEN_SOURCE_EXTENDED` isn't defined, but it can be easily added).
Moreover you can enable audio support, setting the variable `SUPPORT_AUDIO` to "1" (`make SUPPORT_AUDIO=1`).

NOTE: this makefile can live beside autotools without any apparent problem.