To avoid the burden of maintaining multiple build systems, this
patch removes autotools support.

https://bugzilla.gnome.org/show_bug.cgi?id=793340

The website value is inmutable and might be used in the whole source
code. This value has been moved to the `PACKAGE_URL` global variable
that has been usually used in autotools to hold this value.

https://bugzilla.gnome.org/show_bug.cgi?id=793340

The standard place following Vala recommendations[0] for the
`config.vapi` file is the `vapi` directory.

The `config.vapi` file has been moved to the `vapi` directory, and
both meson and autotools have been modified to handle it properly.

https://bugzilla.gnome.org/show_bug.cgi?id=793340

[0] https://wiki.gnome.org/Projects/Vala/GameDevelopmentSeries/Setup

The `config.h` file handling, which is done through the
`config.vapi` file, has been changed following Vala
recommendations[0].

To do this, the global variables in the `config.vapi` file has been
moved to the `Config` namespace.

The `DATADIR` and `LOCALEDIR` variables, although they still exist
in the `config.vapi` file, they are not set in the `config.h` file
because they will be set by the compiler's macro definitions when
using compiler flags.

https://bugzilla.gnome.org/show_bug.cgi?id=793340

[0] https://wiki.gnome.org/Projects/Vala/GameDevelopmentSeries/Setup