Conditional tokens for distro and version

[Shaun McCance <shaunm gnome org>]

I just added support for querying the distro for conditional processing
in Yelp:

https://git.gnome.org/browse/yelp/commit/?id=176c25

This looks at the os-release file (if present) and sets platform tokens
for the distro id and the distro id+version. On the computer I'm typing
this email on, it sets:

platform:fedora
platform:fedora-21

The same code ought to set tokens to check with the os attribute in
DocBook. I only tested Mallard though.

Question time. The os-release file can contain an ID_LIKE field that
specifies other distro the system behaves sort of like:

http://www.freedesktop.org/software/systemd/man/os-release.html#ID_LIKE=

For example, on my CentOS server, there's this:

ID="centos"
ID_LIKE="rhel fedora"
VERSION_ID="7"

With the current code, you get platform:centos and platform:centos-7.
Should you also get platform:rhel and platform:fedora? Or maybe build
the "like"-ness into the token, e.g. platform:like:rhel and
platform:like:fedora?

What's most useful?

--
Shaun