Ported to GTK3
Ignore `config.h.in` generated backup
Enable silent rules
Fix typo in configure.ac
Code style
Add option to build against GTK2

By default gsimplecal uses GTK3 now, but if you pass `--enable-gtk2` to
`./configure` script, it will use GTK2 instead.

Fixes #15.
Do not use procfs for Unique thingy

Some time ago I learned that FreeBSD does not have procfs mounted by
default out of the box, so gsimplecal doesn't work there. So I decided
to drop the procfs stuff and just use `argv[0]` instead for `ftok`.

This also simplifies autotools config since we don't need to check OS to
conditionally set procfs link location. This is a good thing.

Hopefully this doesn't break things for users.

Fixes #17.
Update copyright year