configure: Automatically select GTK+ 3.0 if GTK+ 2.0 is unavailable

configure: Automatically select GTK+ 3.0 if GTK+ 2.0 is unavailable

The configure option --with-gtkabi=3.0 is still supported, but no longer
needed when GTK+-2.0 is missing. When no GTK+ ABI is selected by the
user, configure first tries 2.0, then 3.0.

For some platforms (e.g. Windows) newer binaries of GTK+ are only
available for GTK+ 3.0. Now building on these platforms is a little bit
easier.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>