Lightning: Support reproducible/deterministic builds

[cdecker]

Reproducible builds help close the loop and assert that the binary releases match the source code they were built from. Not sure how hard this is going to be but it's a great assurance for our users.

[jsarenik]

I've just tried to compile three times from scratch and three times I got the same result of sha256 checksum of lightning/lightningd. Is this enough to call the build reproducible/deterministic?

[ZmnSCPxj]

@jsarenik : https://reproducible-builds.org/

[fanquake]

Was this fixed, or no longer an interest?

[cdecker]

We have had reproducible builds for quite some time, I'm just cleaning up some old issues :-)

Reprobuilds are only available for ubuntu:16.04 and amd64, but that'll cover 90% of our users.