Detect timezone via /etc/timezone with /etc/localtime fallback

[nabeya11]

The Makefile previously read /etc/timezone to decide whether to use the
Japanese Fedora mirror. That file is not present on all systems (notably
many systemd-based distros that only manage the timezone via timedatectl),
so the check would silently fail.

Read /etc/timezone first, falling back to resolving the /etc/localtime symlink when it is absent.

[at-wat]

timedatectl doesn't work on WSL environment

$ timedatectl show -p Timezone
System has not been booted with systemd as init system (PID 1). Can't operate.
Failed to connect to system scope bus via local transport: Host is down

I think it's better to put /etc/timezone manually.

[nabeya11]

How about reading the /etc/localtime symlink instead, like readlink -f /etc/localtime 2>/dev/null | sed 's|.*/zoneinfo/||'?
I'm working on Fedora43, which doesn't have /etc/timezone.

[at-wat]

Using /etc/localtime should be ok in most case. (It may be a copy of /usr/share/zoneinfo/*/* file though)
I usually put /etc/timezone manually which is a simple text file.

[nabeya11]

I'll prefer /etc/timezone and fall back to /etc/localtime.