Allow to override build date

[bmwiedemann]

in order to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

This date call is designed to work with both GNU date and BSD date.

[nicowilliams]

Why don't we just get rid of the Date altogether?

[nicowilliams]

Actually, for releases we shouldn't want the USER, the HOST, nor the DATE.

The only things of interest are:

• the actual version
• the HEAD commit hash

[nicowilliams]

How about we drop all of USER, HOST, and DATE? We don't need an option to keep any of those. Just remove them.

[bmwiedemann]

Closing after this was solved slightly differently in master with
6d27e00
469366d

[nicowilliams]

@bmwiedemann Was my fix sufficient for your purporses?

[bmwiedemann]

@nicowilliams it was sufficient, even though I did not set the SOURCE_HOST variable and was building with different hostnames - that is the part I did not understand, but looking into the build VM's filesystem I see

VERSION_HIDDEN const char *heimdal_long_version = "@(#)$Version: Heimdal 7.4.0 by abuild on localhost (x86_64-suse-linux-gnu) 2017-08-03T12:00:00Z $";