README: remove check for automake version in example

It's usually not required, so let's not show a bad example

https://bugzilla.gnome.org/show_bug.cgi?id=741891
gnome-autogen: infer PKG_NAME from configure.ac

Let's not repeat ourself in autogen.sh, there are already many places
with the package name.

https://bugzilla.gnome.org/show_bug.cgi?id=741891
gnome-autogen: warn for deprecated variables

Well, many people still use deprecated variables for 10y, maybe
it's time to let them know.

https://bugzilla.gnome.org/show_bug.cgi?id=741891
gnome-autogen: warn if $srcdir is undefined

Not setting $srcdir will lead to broken out-of-tree autogen call, so
let's ask to set it

https://bugzilla.gnome.org/show_bug.cgi?id=741891