alternative approach

[samdeane]

Some thoughts about an alternative approach:

Running builder should:

• look for a known llbuild file, generate it if it's missing
• execute the llbuild file

If the llbuild file needs (re)generating, it's done by:

• build & execute a product in the root package with a known name (eg: "Configure")
• this outputs the llbuild file

The llbuild file contains enough information to:

• spot that it itself needs regenerating, and to do so in that case
• (re)generate a (hidden) Package.swift file which is used to do the actual build
• build & invoke any pre-compile tools
• invoke swift build on the generated Package.swift, with the correct flags
• invoke any post-compile tools

The advantages of this approach:

• having an llbuild file at the top level enables full dependency checking
• generating a Package.swift for the actual build allows dynamic dependencies to be added (eg platform-specific)