Don't explicitly depend on python 3, so we don't need to have python 2
*and* python 3 installed to build osmo-gsm-manuals.
The script is short and works fine with either python version.
Related: OS#3899
Change-Id: I8af9b8159f5c7e39b905f85edd1584cb4d5a33ef
Print "unknown" to stdout when unix-time-to-fmt.py was called without
an argument, instead of printing a Python stack trace to stderr. It gets
called that way when building outside of a git folder.
Change-Id: I3ba27cc23543e47cc8a1e494c35cc33f1dc8f297
On FreeBSD, the 'date' command's -d option has a completely different meaning.
Instead, use a small python script to do the date format conversion, which
should be more portable.
As a side effect, we now also use UTC instead of the build server's timezone,
which may be considered a more international choice.
Add build/unix-time-to-fmt.py, call in build/Makefile.asciidoc.inc.
Change-Id: I91a40656184f553ee375216d8ba5c7788fe9990d