cmake: EXTERNAL_MPGEN cleanups

[ryanofsky]

Suggested by hebasto in #142 (review)

[theuni]

Hmm, I purposefully avoided namespacing this because I was assuming that parent projects would always want to expose the same option and have it be automatically inherited here. Now they'll have to map it.

Not a big deal to me either way, I guess.

[ryanofsky]

For reference, code which sets this in depends is implemented in bitcoin/bitcoin@99b7b85 from bitcoin/bitcoin#31741 and seems to work reasonably well

[theuni]

For depends, sure, it doesn't matter. But when doing a non-depends cross-build, ccmake shows:

CAPNPC_CXX_EXECUTABLE
CAPNP_EXECUTABLE
...
Libmultiprocess_EXTERNAL_MPGEN

which is rather inconsistent and non-obvious to builders imo. Seems to me Capnp has it right. For a Core, I wouldn't expect to see that namespace.

[ryanofsky]

I see. This suggestion just came from #142 (comment), and I'm happy to rename this to something else or make a different change. Given the other names maybe MPGEN_EXECUTABLE would be a good name for this.

[ryanofsky]

@hebasto might have more ideas too.

[theuni]

I see. This suggestion just came from #142 (comment), and I'm happy to rename this to something else or make a different change. Given the other names maybe MPGEN_EXECUTABLE would be a good name for this.

Fwiw, I originally picked EXTERNAL_CAPNP to mirror (what I thought was) the way Capnp handles it. But I see now that Capnp's is just a bool and CAPNP_EXECUTABLE is the actual path variable. So ACK to MPGEN_EXECUTABLE instead if we go down this path.

[hebasto]

So ACK to MPGEN_EXECUTABLE instead if we go down this path.

So do I.

[theuni]

I attempted this a bunch of different ways, and with some approach this was necessary, but I forget why. I assume this is fine, thanks for simplifying.

[theuni]

Thank you for the cleanups!