Link Shaderc Dynamically

[freesig]

I'm opening this issue to investigate the possibility of linking shaderc dynamically.
It's by far the biggest time consumer in nannous build times.
But all it's doing is compiling a c library which we should be able to install as a system library and just link against.
Things to consider:

• How do we build it the first time? Perhaps we make a small crate that compiles and installs it as a system library.
• How to check if it's already installed. I think maybe the same crate could check if it's already installed and only compile if it isn't. Something similar to moltenvk_deps.
• What if shaderc get's an update. We might need a way to check if this happens and recompile. I'm not sure how this is currently handled. I think it's pulling the source as a submodule from git.
• How to tell vulkano not to just compile shaderc. (And any other crates that use it) We might need a PR to vulkano as well but maybe there is someway to tell shaderc-rs to check for system libraries first.

I'll try and get to this soon but I'm pretty busy with other stuff. This would be a good issue for anyone with cmake experience and I'm happy to help with the build.rs stuff. Probably not a good first issue.

I think the best place to start is to see if there is a way to achieve this with the current setup by passing a feature flag to shaderc-rs or something similar.

[freesig]

I just noticed that libshadec_shared.dylib is distributed with the vulkan_sdk. So this means we automatically have it on mac.

[freesig]

This is possible now with the latest vulkano master and will be in the next vulkano release.
You need to set the SHADERC_LIB_DIR env var to a path where the shaderc lib is.
On mac it is:
SHADERC_LIB_DIR=~/vulkan_sdk/macOS/lib/
Maybe we should look at automating this so that if it's not set it can be set.

[freesig]

I've noticed that this is leading to 500mb+ .rlib files being created multiple times even if you are just using --release.
I'd like to investigate this before closing this issue. I don't know too much about what an rlib is or if that is actually needed. I would have thought we could just link dynamically to the shaderc library without needing to make any large files like this.