Add cross-compilation options only when requested

[jsarenik]

Requesting them is done by setting BUILD=<target_arch>

Otherwise autotools (used by external dependencies like
libsodium) is not happy with setting cross-compilation
variables and may possibly lead to unexpected results.

Changelog-None

[jsarenik]

Sorry about this, I held it for too long. It was fresh long time ago, when I first started poking into c-lightning. Happy to see it unresolved yet, I decided to send this PR :)

[rustyrussell]

Ack 6d9471c

[cdecker]

Thanks @jsarenik for the PR. Seems quite all right but I'd like to get @rustyrussell's sign-off since he's our resident Makefile expert. (need to read the comments before commenting myself...)

One thing that seems a bit strange is the naming of the variables. I think TARGET might be a better name for the BUILD variable, and the MAKE_HOST_ADD might be better called CROSSCOMPILEOPTS since we usually have names like XYZOPTS for things that get added to the command lines.

[jsarenik]

Thanks @jsarenik for the PR. Seems quite all right

One thing that seems a bit strange is the naming of the variables. I think TARGET might be a better name for the BUILD variable, and the MAKE_HOST_ADD might be better called CROSSCOMPILEOPTS since we usually have names like XYZOPTS for things that get added to the command lines.

Sure. Thank you for feed-back. I agree with CROSSCOMPILEOPTS, but the TARGET is a bit tricky. For one thing, it was BUILD variable used in c-lightning since some time ago so such a change could be considered as a regression for some. And BUILD also seems correct from other point of view:

See Autoconf documentation on this topic which nicely explains how it is meant to be. Generally, I would change even more to not use the $MAKE_HOST variable by not setting --host at all. Just --build as the documentation clearly explains:

They all default to the result of running config.guess, unless you specify either --build or --host. In this case, the default becomes the system type you specified. If you specify both, and they're different, configure enters cross compilation mode, so it doesn't run any tests that require execution.

So now I will change the patch from MAKE_HOST_ADD to CROSSCOMPILEOPTS and wait for @rustyrussell to decide on whether to leave --host together with ${MAKE_HOST} or whether to drop it. I am all for dropping it and leaving only --build and will happily do all the needed testing to (dis)prove it.

Just for reference: Another solid source from GNU Automake docs

[jsarenik]

Just a note: doc/INSTALL.md might need to be changed, I am going to test it all now. First unchanged, just following the docs.

[cdecker]

Great, thanks @jsarenik, I hadn't considered that BUILD is already part of the external interface, thus changing would be breaking things 👍

[jsarenik]

...I hadn't considered that BUILD is already part of the external interface, thus changing would be breaking things

Maybe it is not really. It is referenced only once in the doc/INSTALL.md.

[jsarenik]

To refresh memories, I went through #1533

[cdecker]

ACK 635a9d7