Add configure option --enable-build-timestamp.
* configure.ac (BUILD_TIMESTAMP): Set to "<none>" by default.
This is based on
libgpg-error commit d620005fd1a655d591fccb44639e22ea445e4554
but changed to be disbaled by default. Check there for some
background.
- Signed-off-by: Werner Koch <wk@gnupg.org>