Added documentation for SPI_PROCESSING

[daveverwer]

In the "Common Use Cases" article. It's not ideal here as this article says it's specifically about the .spi.yml file, but I think it's OK as this is where we talk about the build system.

[finestructure]

I'll add something about SPI_BUILD as well, because we still need this independently from SPI_PROCESSING.

swift-java is short-circuiting JAVA_HOME discovery in order to make dump-package work during validation and analysis on machines that don't have Java installed.

However, in order to actually build the package on a machine the has Java installed, it will have to conditionally re-enable it by skipping the short-circuit if SPI_BUILD is set.

Since it's unlikely we'll ever ship an analysis or validation image with Java installed we really need both variables to offer this kind of control to package authors.

[finestructure]

It's actually not a critical distinction to have SPI_BUILD set but it makes for a clearer error message in swift-java's build failures until we ship builders with Java installed: swiftlang/swift-java#347

[daveverwer]

The additions look good to me. Merge if you're happy with them. 👍

[finestructure]

I'd accidentally pushed it to the existing branch instead of a new one. Hard pushed it back and opened a PR instead now!

[finestructure]

Hold on, CI is failing

[daveverwer]

I thought that was a persistent issue on this repo, it's certainly not related to documentation changes.

It's the setup-swift step that's failing.

[finestructure]

Yeah, it's clearly not change related but the latest release (1.10.0) did pass CI, so something must have changed in the meantime. I'll take a look.