Makefile: sail can report its library path

[nwf]

And sail has binary releases, so it may not be installed in a way opam understands. Therefore, apply the relevant hunk from riscv/sail-riscv#532 so that we just ask the sail that we find on the $PATH where it was installed, rather than asking opam.

[nwf]

Sigh. 0.17 spells this -dir and 0.18 spells it --dir but accepts -dir; upstream has already moved to 0.18. Let's try that again.

[rmn30]

Interesting. I didn't know about the -dir argument.

One thing I don't like is that if I want to build using a local checkout / build of sail I can no longer set SAIL_DIR in my environment and have it just work. I think I have to either:

1. add the locally built sail early in my PATH or
2. specify SAIL=... on the command line to make every time.

I guess either of these is OK but it will break any workflow that was relying on setting SAIL_DIR previously.

I am also concerned that the setting of LEM_DIR will only work if lem happens to be installed adjacent to sail. That might be the case if lem and sail are installed by opam but isn't universal. I don't think anyone actually uses the lem output anyway so it probably doesn't matter...

[nwf]

Yeah, my workflow involves PATH=/path/to/sail/bin:$PATH make, which leaves something to be desired, to be sure, but I think I'd like us to keep alignment with upstream. (It might be nice to redo their Makefile so that it was just the list of source files and then an include of a common infrastructure that we could reuse.)