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.
Also use UTC to be independent of timezone.

This date call is designed to work with all variants of 'date'

Without this patch, the dosemu package in openSUSE would differ for every build.

[stsp]

What does @$SOURCE_DATE_EPOCH mean here?
And too many bashisms, please write as
if test -n "$SOURCE_DATE_EPOCH"

[bmwiedemann]

The @ is not shell magic, but a literal @ (at) sign as documented in the GNU date man page

EXAMPLES
       Convert seconds since the epoch (1970-01-01 UTC) to a date

              $ date --date='@2147483647'```

[stsp]

So what you do with -r here?
Your doc does not say this can be a file,
or what is the intention?
Can we just have

CONFIG_TIME=`date -u -d "@$SOURCE_DATE_EPOCH" "+$DATE_FMT"`

and nothing else in this line?

[bmwiedemann]

date -d would only work with GNU date. BSD date (e.g. MacOSX) takes such integer dates with the -r option instead. And solaris date has neither - that is covered by the third case.

[stsp]

OK, thanks!