Bumping GLib Requirement for Pidgin3

[Ethan Blanton]

Jorge Villaseñor spake unto us the following wisdom:
> Looking at https://developer.pidgin.im/wiki/Dependencies I would say that
> for 3.0 we bump the glib version to 2.40 (included in RHEL 7) and leave old
> versions of distributions to live with 2.x.y or backport if they want to.

2.40 would include Ubuntu LTS, Debian Jessie, and RHEL 7.  I have no
problem with dropping support for long-term releases older than these.
By the time Pidgin 3.0 is out, they will all be thoroughly established
(Jessie is, to be fair, only a few days old!) and supported for many
years to come.

I have no problem with this bump, particularly if there's anything in
2.40 that we want to use.  Whatever cruft we keep around now is likely
to be with us for quite a while!

Ethan