New CLI parameter: --c-source [options] [file]
It even works with `--cache on` when there are transitive dependencies.
Instead of `builder.addCExecutable`, use `builder.addExecutable` and pass
`null` for the root source file. Then use `builder.addCSourceFile`,
which takes the path to the C code, and a list of C compiler args.
Be sure to linkSystemLibrary("c") if you want libc headers to be
available.
Merge TestStep into LibExeObjStep. That was long overdue.
e76ce2c1d0