misc: added libconfig as a subproject

[Monsterovich]

No description provided.

[yshui]

git submodule in general is a pain with work with, so I try to avoid them when possible. and I do want to push debian a little bit to update their libconfig. I am not objecting to adding this but, I would rather prefer either:

1. a meson wrap file, or
2. instructions in README about how to build libconfig.

[Monsterovich]

@yshui

1. a meson wrap file, or

Done, I went for a master branch because tag v1.7.3 doesn't build, most likely due to a bug in the CMake files which they fixed later.

[absolutelynothelix]

also it'd be good to squash all this merge commit madness. i personally think it could be a single commit but it's up to you (and yshui).

[Monsterovich]

also it'd be good to squash all this merge commit madness. i personally think it could be a single commit but it's up to you (and yshui).

Done.