Allow to compile rust-builder as a static c library with the builder calls exported

[Makogan]

I would find it useful if rust-builder could be compiled as a static library that could then be invoked from rust, C or C++. This has some possible advantages:

• Allowing a project using a new toolchain to use rust builder
• Allowing non rust projects to use rust shaders

[rust-gpu-bot]

This issue is now being tracked at: Rust-GPU/rust-gpu#58