`copilot-c99`: Add volatile qualifier to external stream variable

[robdockins]

Minor enhancements to the code generation process. This adds volatile qualifiers to external stream global variables, and allows users to select an output directory for generated code.

[ivanperez-keera]

@robdockins the feature to customize the output directory is already in copilot. Can you please rebase this PR wrt. develop, dropping that commit and leaving only 6de857e2347d4f172f1316a5e2037b6ecc3c3f03?

No other changes are needed; if they are, I will ask.

Thanks!

[robdockins]

Will do

[robdockins]

Rebased as requested.