Makefile.am: Use SOURCE_DATE_EPOCH environment variable

Makefile.am: Use SOURCE_DATE_EPOCH environment variable

This package defines PKGBLDDATE as build timestamp which makes
it non reproducible. Use SOURCE_DATE_EPOCH if it is found, otherwise
use build timestamp. Following best practices, see :
https://reproducible-builds.org/docs/source-date-epoch/

The patch is BSD compatible too.

Change-Id: I26c1a00f2e8059ae31fe72a794b5962af5a84f44
Co-developed-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Sofiane HAMAM <sofiane.hamam@smile.fr>
Reviewed-on: https://review.openocd.org/c/openocd/+/8619
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Tested-by: jenkins