Allow to override build date
filed https://sourceforge.net/p/clisp/feature-requests/59/
timestamps beyond 2036 do not work
It is LISP, which uses a 32-bit unsigned integer to count seconds since 1900-01-01. date -u -d 1900-01-01 +%s -2208988800 2**32 - 2208988800 => 2085978496 date -u -d @2085978496 2036-02-07T06:28:16 UTC
Allow to override build date with SOURCE_DATE_EPOCH
xindy fails to build after 2036-02-07
Alternatively, we could use the POSIX template from https://reproducible-builds.org/docs/source-date-epoch/ to allow to override the date with SOURCE_DATE_EPOCH- but if we want to support date implementations other than GNU date, this increased code complexity a bit.
Drop build date to make package build reproducible