Disabling compiling from source

Could you add a feature to disable building from source? Compiling shaderc can take several minutes and there is no indication in cargo output why the build is hanging. I would prefer to fail instead if a pre-compiled library is not available.

Start by tracing the repository's shaderc build path and how Cargo selects a pre-compiled library. Determine where an option can disable source compilation and what failure should be reported when no pre-compiled library is available. Done means the option reliably prevents source builds and fails clearly in that case.

Written by the indexing model from the issue text.