[Linux/BSD] Include headers for dynamically loaded libraries to simplify build dependencies.

[bruvzg]

Includes headers and licensing information for the libraries wrapped by https://github.com/hpvb/dynload-wrapper and dynamically loaded.

• Simplify build dependencies, since libs are loaded dynamically, there's no reason to have the full library (and all of its dependencies) on the system used to build.
• Ensures that wrappers and headers have the same version, Ubuntu 18.04 LTS (bionic) versions are used.
• Adds missing licensing and version information for dynamically loaded libraries.

Fixes #67863 (and similar issues with build environment setup).

[hpvb]

This is a good idea! I like it.

[Riteo]

I wonder if at this point dynload-wrapper could make (at least almost) all-in-one headers for that, GLAD style.

[Riteo]

Why isn't X11 in there? It's dynamically loaded now.

Edit: note that if you try to regenerate the wrappers you'll stumble on a lot of issues, so I'd recommend not touching them for now.

[akien-mga]

I think this is great!

The only thing I'm unsure of is the whole license/copyright documentation situation. If we're vendoring the headers it definitely makes sense to include a license file next to the headers to make it clear what terms this specific code is under.

But I don't see how using library headers from a vendored copy, or the ones provided on the build environment, would differ in practice - the same header code gets included in the Godot binary. So if we weren't documenting the license terms of all dlopen'ed libraries or system headers used by Godot code in COPYRIGHT.txt before, I don't think we ought to document them now.

It does make me doubt whether we should actually have been documenting these licenses all along, and that's something I'll try to get clarified with open source licensing experts. If we have to, I've basically never seen a project do this ever, whether open source or proprietary, so this would be an interesting situation.

So for this PR I'd suggest:

• Move all Linux library headers in the same folder to avoid adding too many folders in thirdparty. Maybe thirdparty/linuxbsd_headers/.
• Document the versions etc. of those libraries in thirdparty/README.md all together under the ## linuxbsd_headers section. Can be kept simple like for the ## misc section since the update instructions are trivial and don't need to be mentioned.
• Don't list them in COPYRIGHT.txt.

As a side note, would be worth checking if we really needed to include the full set of headers or if we can get away with only a subset (i.e. the ones we include directly + their dependencies, if that's not the whole set).

[bruvzg]

Added X11 headers, moved headers to thirdparty/linuxbsd_headers/ alongside with own README file to list versions, and removed from COPYRIGHT.txt.

[akien-mga]

Thanks!