Allow users of IUI to specify ui-sys features build and fetch

[porky11]

Build and fetch failed for me to build, and since it's more common to install c libraries oneself before using in rust, these features should be disabled by default.

[NoraCodes]

since it's more common to install c libraries oneself before using in rust

Citation needed.

Specifically for libui, we're interested mostly in providing a smooth-as-possible experience for starting a project; while there are dependency issues to work out (CMake, platform libraries, etc), I think at the moment the best thing to do is to keep things the way they are, especially since we depend on a fairly arbitrary version of libui at the moment.

Thanks for bringing this to our attention, though!

[porky11]

The main problem was, that I couldn't easily change the default features, since they were not changable in the more rusty iui wrapper

[NoraCodes]

This is a good point, and I'm sorry I misunderstood the problem originally. The answer is, I think, to propagate these features up to the IUI level.

[hedgehog1029]

Also running into this (edited) issue, it doesn't seem to be possible for me, a consumer of iui, to disable build & fetch via my cargo.toml