CLIENT_DATE has fake date in gitian builds #3570

The idea is to set the CLIENT_DATE (BUILD_DATE in share/genbuild.sh to the date of last commit.

However, the problem with gitian builds:

• gitian builds are built from a make dist archive and not from git directly
• share/genbuild.sh tries to get information from git, if this fails, it uses the current date (which is useless in gitian bulds)

This is tricky to fix. The most general solution would be to generate and package a build.h on make dist instead of trying to generate it without git information when building.

Note: The build date is only shown in the GUI debug window, as well as logged in debug.log and printed in wallet exports, so this is low priority, but can potentially confuse users.

Seems this has been fixed many times over 0.9 history:

21cc8bdcbea19ca961994ddc19a6f97b4a347fb8 Fix build date for from-tarball builds ef1e984eade3166f807197eccba378fa937b6f88 Dmitry Smirnov

I think the last 'fix' is what actually broke behavior again. Using date makes the build non-deterministic.