build: improve reliability/portability of date command usage (#6404)HEADmaster

Changes:

* Use the exact same source date string for all `date` invocations
* Use `-d` instead of `--date=`
* Fallback to `-r` and then to no argument

Some `date` implementations only support BSD `-r` instead of GNU `-d` /
`--date=` and others may not support any of them since neither option is
in POSIX.

For example, if zoneinfo is installed by chromebrew on ChromeOS, it
provides a date program that only supports `-r` and overrides the system
one (which supports `-d`) [1]:

    ./mkman.sh 0.9.72 src/man/firejail.man firejail.1
    date: invalid option -- '-'
    date: usage: date [-u] [-c] [-r seconds] [+format]
    make: *** [Makefile:42: firejail.1] Error 1

Environment: zoneinfo 2024a on ChromeOS M125.

Note: The changes are based on what is suggested by
reproducible-builds.org [2].

Relates to #193.

Fixes #6403.

[1] https://github.com/netblue30/firejail/issues/6403#issue-2402292506
[2] https://reproducible-builds.org/docs/source-date-epoch/

Reported-by: @Zopolis4

0 files changed, 0 insertions, 0 deletions