Switch to meson build

[MOZGIII]

libui switched from cmake to meson, so this project should keep up.

I decided to create this - a tracking issue on thoughs and desired features for that change.

While switching to meson build system, it would be great to have support for the following things, or at least keep them in mind:

1. The ability to control whether to build a static or dynamic lib. Apparently, the recommended way of doing that is via build-time env vars, and not via cargo features.
2. The ability to embed custom manifests on windows.

Thoughts?

[MOZGIII]

https://github.com/MOZGIII/libui-sys - I'll just leave it here...

[mqudsi]

Meson has a hard dependency on python3 that is never going away; something to keep in mind.

[MOZGIII]

I've implemented libui-sys that's compatible with ui-rs from this repo. This libui-sys implementation is not a fork, built from scratch, but is a drop-in replacement for the ui-sys that this repo hosts. I also have a fork of libui-rs - that is patched to use my libui-sys crate.

I'd be happy to integrate my changes back if there's an interest from the community. For now, I'm just using those myself in my rebootinto project.

[phase]

@MOZGIII's fork worked perfectly for me! I would love to see this merged in.

[xobs]

I just saw this issue now.

I put together #86 that does away with the libui build system entirely and directly includes the source files by building them using cc-rs. So far I've tested it on Windows (MSVC) and Unix (Ubuntu 16.04).

It's simple enough to pull out a list of files to include, and the approach I've taken could be adapted to integrate with the newer meson build system.