Allow for multiple GDK backends
Allow for multiple GDK backends

In GDK > 3.0, multiple backends can be built. The pkg-config
variable that tells the backend is now called 'targets' instead of
'target'. To determine whether to build for OS X, configure now
needs to use the GTK_CHECK_BACKENDS macro supplied with GTK, as
described in the migration guide:
http://developer.gnome.org/gtk3/3.3/ch25s02.html

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